Check for duplicate flags.

Check that the short and long flag of each option has not been
defined before adding it to the group. Returns an ErrDuplicatedFlag
in case of error.

error.go

@@ -34,6 +34,9 @@ const (
 
 	// A short flag name is longer than one character
 	ErrShortNameTooLong
+
+	// A short or long flag has been defined more than once
+	ErrDuplicatedFlag
 )
 
 func (e ErrorType) String() string {
@@ -54,6 +57,8 @@ func (e ErrorType) String() string {
 		return "no argument for bool"
 	case ErrRequired:
 		return "required"
+	case ErrDuplicatedFlag:
+		return "duplicated flag"
 	}
 
 	return "unknown"

group_private.go

@@ -161,6 +161,27 @@ func (g *Group) scanStruct(realval reflect.Value, sfield *reflect.StructField, h
 			value: realval.Field(i),
 			tag:   mtag,
 		}
+
+		var existingOption *Option
+		if longname != "" {
+			existingOption = g.optionByName(longname, nil)
+			if existingOption != nil {
+				return newErrorf(ErrDuplicatedFlag,
+					"option `%s' uses the same long name as option `%s'",
+					option,
+					existingOption)
+			}
+		}
+
+		if short != 0 {
+			existingOption = g.optionByName(string(short), nil)
+			if existingOption != nil {
+				return newErrorf(ErrDuplicatedFlag,
+					"option `%s' uses the same short name as option `%s'",
+					option,
+					existingOption)
+			}
+		}
 
 		g.options = append(g.options, option)
 	}