Fix:(issue_1500). Fix slice flag value duplication issue

flag.go

@@ -126,6 +126,21 @@ type Flag interface {
 	RunAction(*Context) error
 }
 
+// DocGenerationFlag is an interface that allows documentation generation for the flag
+type DocGenerationFlag interface {
+	Flag
+
+	// TakesValue returns true if the flag takes a value, otherwise false
+	TakesValue() bool
+
+	// GetValue returns the flags value as string representation and an empty
+	// string if the flag takes no value at all.
+	GetValue() string
+
+	// GetEnvVars returns the env vars for this flag
+	GetEnvVars() []string
+}
+
 // DocGenerationSliceFlag extends DocGenerationFlag for slice-based flags.
 type DocGenerationSliceFlag interface {
 	DocGenerationFlag
@@ -293,8 +308,14 @@ func formatDefault(format string) string {
 }
 
 func stringifyFlag(f Flag) string {
-	placeholder, usage := unquoteUsage(f.GetUsage())
-	needsPlaceholder := f.TakesValue()
+	// enforce DocGeneration interface on flags to avoid reflection
+	df, ok := f.(DocGenerationFlag)
+	if !ok {
+		return ""
+	}
+
+	placeholder, usage := unquoteUsage(df.GetUsage())
+	needsPlaceholder := df.TakesValue()
 
 	if needsPlaceholder && placeholder == "" {
 		placeholder = defaultPlaceholder
@@ -302,7 +323,7 @@ func stringifyFlag(f Flag) string {
 
 	defaultValueString := ""
 
-	if s := f.GetDefaultText(); s != "" {
+	if s := df.GetDefaultText(); s != "" {
 		defaultValueString = fmt.Sprintf(formatDefault("%s"), s)
 	}