diff --git a/flagutil.go b/flagutil.go
index 08e2b55..a048790 100644
--- a/flagutil.go
+++ b/flagutil.go
@@ -37,14 +37,14 @@ func (fn PrinterFunc) Name(ctx context.Context, fs parse.FlagSet) (func(*flag.Fl
 
 type parser struct {
 	Parser
-	stash           func(*flag.Flag) bool
-	ignoreUndefined bool
+	stash               func(*flag.Flag) bool
+	ignoreUndefined     bool
+	allowResetSpecified bool
 }
 
 type config struct {
 	parsers          []*parser
 	parserOptions    []ParserOption
-	ignoreUndefined  bool
 	customUsage      bool
 	unquoteUsageMode UnquoteUsageMode
 }
@@ -67,16 +67,12 @@ func buildConfig(opts []ParseOption) config {
 func Parse(ctx context.Context, flags *flag.FlagSet, opts ...ParseOption) (err error) {
 	c := buildConfig(opts)
 
-	fs := parse.NewFlagSet(flags,
-		parse.WithIgnoreUndefined(c.ignoreUndefined),
-	)
+	fs := parse.NewFlagSet(flags)
 	for _, p := range c.parsers {
 		parse.NextLevel(fs)
 		parse.Stash(fs, p.stash)
-		if p.ignoreUndefined {
-			// User may ask to ignore undefined per parser as well.
-			parse.IgnoreUndefined(fs)
-		}
+		parse.IgnoreUndefined(fs, p.ignoreUndefined)
+		parse.AllowResetSpecified(fs, p.allowResetSpecified)
 
 		if err = p.Parse(ctx, fs); err != nil {
 			if err == flag.ErrHelp {
diff --git a/options.go b/options.go
index d58b369..613f961 100644
--- a/options.go
+++ b/options.go
@@ -26,13 +26,17 @@ type ParseOptionFunc func(*config)
 
 func (fn ParseOptionFunc) setupParseConfig(c *config) { fn(c) }
 
-type ParserOptionFunc func(*config, *parser)
+type ParserOptionFunc func(*parser)
 
-func (fn ParserOptionFunc) setupParseConfig(c *config)  { fn(c, nil) }
-func (fn ParserOptionFunc) setupParserConfig(p *parser) { fn(nil, p) }
+func (fn ParserOptionFunc) setupParserConfig(p *parser) { fn(p) }
 
-func stashFunc(check func(*flag.Flag) bool) (opt ParserOptionFunc) {
-	return ParserOptionFunc(func(c *config, p *parser) {
+type ParseOrParserOptionFunc func(*config, *parser)
+
+func (fn ParseOrParserOptionFunc) setupParseConfig(c *config)  { fn(c, nil) }
+func (fn ParseOrParserOptionFunc) setupParserConfig(p *parser) { fn(nil, p) }
+
+func stashFunc(check func(*flag.Flag) bool) (opt ParseOrParserOptionFunc) {
+	return ParseOrParserOptionFunc(func(c *config, p *parser) {
 		if c != nil {
 			c.parserOptions = append(c.parserOptions, opt)
 			return
@@ -47,24 +51,30 @@ func stashFunc(check func(*flag.Flag) bool) (opt ParserOptionFunc) {
 	})
 }
 
-func WithStashName(name string) ParserOptionFunc {
+func WithStashName(name string) ParseOrParserOptionFunc {
 	return stashFunc(func(f *flag.Flag) bool {
 		return f.Name == name
 	})
 }
 
-func WithStashPrefix(prefix string) ParserOptionFunc {
+func WithStashPrefix(prefix string) ParseOrParserOptionFunc {
 	return stashFunc(func(f *flag.Flag) bool {
 		return strings.HasPrefix(f.Name, prefix)
 	})
 }
 
-func WithStashRegexp(re *regexp.Regexp) ParserOptionFunc {
+func WithStashRegexp(re *regexp.Regexp) ParseOrParserOptionFunc {
 	return stashFunc(func(f *flag.Flag) bool {
 		return re.MatchString(f.Name)
 	})
 }
 
+func WithResetSpecified() ParserOptionFunc {
+	return ParserOptionFunc(func(p *parser) {
+		p.allowResetSpecified = true
+	})
+}
+
 // WithParser returns a parse option and makes p to be used during Parse().
 func WithParser(p Parser, opts ...ParserOption) ParseOptionFunc {
 	x := &parser{
@@ -79,17 +89,28 @@ func WithParser(p Parser, opts ...ParserOption) ParseOptionFunc {
 }
 
 // WithIgnoreUndefined makes Parse() to not fail on setting undefined flag.
-func WithIgnoreUndefined() ParserOptionFunc {
-	return ParserOptionFunc(func(c *config, p *parser) {
+func WithIgnoreUndefined() (opt ParseOrParserOptionFunc) {
+	return ParseOrParserOptionFunc(func(c *config, p *parser) {
 		switch {
 		case c != nil:
-			c.ignoreUndefined = true
+			c.parserOptions = append(c.parserOptions, opt)
 		case p != nil:
 			p.ignoreUndefined = true
 		}
 	})
 }
 
+func WithAllowResetSpecified() (opt ParseOrParserOptionFunc) {
+	return ParseOrParserOptionFunc(func(c *config, p *parser) {
+		switch {
+		case c != nil:
+			c.parserOptions = append(c.parserOptions, opt)
+		case p != nil:
+			p.allowResetSpecified = true
+		}
+	})
+}
+
 // WithCustomUsage makes Parse() to ignore flag.FlagSet.Usage field when
 // receiving flag.ErrHelp error from some parser and print results of
 // flagutil.PrintDefaults() instead.
diff --git a/parse/flagset.go b/parse/flagset.go
index fe0a9b4..b68e224 100644
--- a/parse/flagset.go
+++ b/parse/flagset.go
@@ -39,22 +39,28 @@ func Stash(fs FlagSet, fn func(*flag.Flag) bool) {
 	fset.stash = fn
 }
 
-func IgnoreUndefined(fs FlagSet) {
+func IgnoreUndefined(fs FlagSet, ignore bool) {
 	fset := fs.(*flagSet)
-	fset.ignoreUndefined = true
+	fset.ignoreUndefined = ignore
+}
+
+func AllowResetSpecified(fs FlagSet, allow bool) {
+	fset := fs.(*flagSet)
+	fset.allowResetSpecified = allow
 }
 
 type flagSet struct {
-	dest            *flag.FlagSet
-	ignoreUndefined bool
-	provided        map[string]bool
-	stash           func(*flag.Flag) bool
+	dest                *flag.FlagSet
+	ignoreUndefined     bool
+	allowResetSpecified bool
+	specified           map[string]bool
+	stash               func(*flag.Flag) bool
 }
 
 func NewFlagSet(flags *flag.FlagSet, opts ...FlagSetOption) FlagSet {
 	fs := &flagSet{
-		dest:     flags,
-		provided: make(map[string]bool),
+		dest:      flags,
+		specified: make(map[string]bool),
 	}
 	for _, opt := range opts {
 		opt(fs)
@@ -64,7 +70,7 @@ func NewFlagSet(flags *flag.FlagSet, opts ...FlagSetOption) FlagSet {
 }
 
 func (fs *flagSet) Set(name, value string) error {
-	if fs.provided[name] {
+	if fs.specified[name] && !fs.allowResetSpecified {
 		return nil
 	}
 	f := fs.dest.Lookup(name)
@@ -92,13 +98,13 @@ func (fs *flagSet) stashed(f *flag.Flag) bool {
 
 func (fs *flagSet) update() {
 	fs.dest.Visit(func(f *flag.Flag) {
-		fs.provided[f.Name] = true
+		fs.specified[f.Name] = true
 	})
 }
 
 func (fs *flagSet) VisitUnspecified(fn func(*flag.Flag)) {
 	fs.dest.VisitAll(func(f *flag.Flag) {
-		if !fs.provided[f.Name] && !fs.stashed(f) {
+		if !fs.specified[f.Name] && !fs.stashed(f) {
 			fn(fs.clone(f))
 		}
 	})