diff --git a/src/main/java/org/eolang/lints/errors/LtObjectIsNotUnique.java b/src/main/java/org/eolang/lints/errors/LtObjectIsNotUnique.java
new file mode 100644
index 00000000..e569be03
--- /dev/null
+++ b/src/main/java/org/eolang/lints/errors/LtObjectIsNotUnique.java
@@ -0,0 +1,88 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2016-2024 Objectionary.com
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included
+ * in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+package org.eolang.lints.errors;
+
+import com.jcabi.xml.XML;
+import java.util.Collection;
+import java.util.LinkedList;
+import java.util.Map;
+import java.util.Objects;
+import org.eolang.lints.Defect;
+import org.eolang.lints.Lint;
+import org.eolang.lints.Severity;
+
+/**
+ * Object is not unique.
+ *
+ * @since 0.0.30
+ */
+public final class LtObjectIsNotUnique implements Lint<Map<String, XML>> {
+
+    @Override
+    public String name() {
+        return "object-is-not-unique";
+    }
+
+    @Override
+    public Collection<Defect> defects(final Map<String, XML> pkg) {
+        final Collection<Defect> defects = new LinkedList<>();
+        pkg.values().forEach(
+            xmir -> {
+                final String src = xmir.xpath("/program/@name").get(0);
+                final String name = xmir.xpath("/program/objects/o[1]/@name").get(0);
+                pkg.values().forEach(
+                    oth -> {
+                        if (!Objects.equals(oth, xmir)) {
+                            final String other = oth.xpath("/program/objects/o[1]/@name").get(0);
+                            if (name.equals(other)) {
+                                defects.add(
+                                    new Defect.Default(
+                                        this.name(),
+                                        Severity.ERROR,
+                                        oth.xpath("/program/@name").stream()
+                                            .findFirst()
+                                            .orElse("unknown"),
+                                        Integer.parseInt(
+                                            oth.xpath("/program/objects/o[1]/@line").get(0)
+                                        ),
+                                        String.format(
+                                            "The object name '%s' is not unique, original object was found in '%s'",
+                                            name, src
+                                        )
+                                    )
+                                );
+                            }
+                        }
+                    }
+                );
+            }
+        );
+        return defects;
+    }
+
+    @Override
+    public String motive() throws Exception {
+        throw new UnsupportedOperationException("#motive()");
+    }
+}
diff --git a/src/test/java/org/eolang/lints/errors/LtObjectIsNotUniqueTest.java b/src/test/java/org/eolang/lints/errors/LtObjectIsNotUniqueTest.java
new file mode 100644
index 00000000..30be12fc
--- /dev/null
+++ b/src/test/java/org/eolang/lints/errors/LtObjectIsNotUniqueTest.java
@@ -0,0 +1,87 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2016-2024 Objectionary.com
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included
+ * in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+package org.eolang.lints.errors;
+
+import com.jcabi.xml.XML;
+import com.jcabi.xml.XMLDocument;
+import org.cactoos.io.ResourceOf;
+import org.cactoos.map.MapEntry;
+import org.cactoos.map.MapOf;
+import org.eolang.parser.EoSyntax;
+import org.hamcrest.MatcherAssert;
+import org.hamcrest.Matchers;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests for {@link LtObjectIsNotUnique}.
+ *
+ * @since 0.0.30
+ */
+final class LtObjectIsNotUniqueTest {
+
+    @Test
+    void catchesDuplicates() throws Exception {
+        ;
+        MatcherAssert.assertThat(
+            "Defects are empty, but they should not",
+            new LtObjectIsNotUnique().defects(
+                new MapOf<String, XML>(
+                    new MapEntry<>(
+                        "foo", LtObjectIsNotUniqueTest.xmir("foo")
+                    ),
+                    new MapEntry<>(
+                        "bar-with-foo", LtObjectIsNotUniqueTest.xmir("bar-with-foo")
+                    )
+                )
+            ),
+            Matchers.hasSize(Matchers.greaterThan(0))
+        );
+    }
+
+    @Test
+    void allowsAllUnique() {
+        MatcherAssert.assertThat(
+            "Defects aren't empty, but they should",
+            new LtObjectIsNotUnique().defects(
+                new MapOf<String, XML>(
+                    new MapEntry<>("foo", new XMLDocument("")),
+                    new MapEntry<>("bar", new XMLDocument(""))
+                )
+            ),
+            Matchers.emptyIterable()
+        );
+    }
+
+    private static XML xmir(final String name) throws Exception {
+        return new EoSyntax(
+            name,
+            new ResourceOf(
+                String.format(
+                    "org/eolang/lints/errors/object-is-not-unique/%s.eo",
+                    name
+                )
+            )
+        ).parsed();
+    }
+}
diff --git a/src/test/resources/org/eolang/lints/errors/object-is-not-unique/bar-with-foo.eo b/src/test/resources/org/eolang/lints/errors/object-is-not-unique/bar-with-foo.eo
new file mode 100644
index 00000000..c1dd8f66
--- /dev/null
+++ b/src/test/resources/org/eolang/lints/errors/object-is-not-unique/bar-with-foo.eo
@@ -0,0 +1,2 @@
+# Bar.
+[] > foo
diff --git a/src/test/resources/org/eolang/lints/errors/object-is-not-unique/bar.eo b/src/test/resources/org/eolang/lints/errors/object-is-not-unique/bar.eo
new file mode 100644
index 00000000..e69de29b
diff --git a/src/test/resources/org/eolang/lints/errors/object-is-not-unique/foo.eo b/src/test/resources/org/eolang/lints/errors/object-is-not-unique/foo.eo
new file mode 100644
index 00000000..24ccef07
--- /dev/null
+++ b/src/test/resources/org/eolang/lints/errors/object-is-not-unique/foo.eo
@@ -0,0 +1,2 @@
+# Foo.
+[] > foo