urfave/cli · error

can't duplicate this flag

Error message

can't duplicate this flag

What it means

BoolWithInverseFlag.Set enforces the OnlyOnce option: if the flag (or its inverse alias) has already been set (count > 0) and OnlyOnce is true, further Set calls return 'can't duplicate this flag'. PostParse invokes Set for sourced values, so a source plus a command-line occurrence can collide.

Source

Thrown at flag_bool_with_inverse.go:119

				val = "false"
			}
			if err := bif.Set(bif.Name, val); err != nil {
				return fmt.Errorf(
					"could not parse %[1]q as %[2]T value from %[3]s for flag %[4]s: %[5]s",
					val, bif.Value, source, bif.Name, err,
				)
			}

			bif.hasBeenSet = true
		}
	}

	return nil
}

func (bif *BoolWithInverseFlag) Set(name, val string) error {
	if bif.count > 0 && bif.OnlyOnce {
		return fmt.Errorf("can't duplicate this flag")
	}

	bif.hasBeenSet = true

	if slices.Contains(append([]string{bif.Name}, bif.Aliases...), name) {
		if bif.nset {
			return fmt.Errorf("cannot set both flags `--%s` and `--%s`", bif.Name, bif.inversePrefix()+bif.Name)
		}
		if err := bif.value.Set(val); err != nil {
			return err
		}
		bif.pset = true
	} else {
		if bif.pset {
			return fmt.Errorf("cannot set both flags `--%s` and `--%s`", bif.Name, bif.inversePrefix()+bif.Name)
		}
		if err := bif.value.Set("false"); err != nil {
			return err

View on GitHub (pinned to 1a4deb4f5a)

Solutions

  1. Remove the duplicate occurrence, keeping one of the flag or its inverse
  2. Set OnlyOnce to false on the flag definition if repetition should be allowed
  3. Unset the mapped env/config source so only the explicit CLI value applies

Example fix

// before
$ app --verbose --no-verbose  // duplicate
// after
$ app --verbose  // one occurrence only
Defensive patterns

Strategy: validation

Validate before calling

count := 0
for _, a := range os.Args[1:] {
    if a == "--verbose" || a == "--no-verbose" || a == "-v" { count++ }
}
if count > 1 { return errors.New("flag given multiple times and OnlyOnce is set") }

Try / catch

if err := cmd.Run(os.Args); err != nil {
    if strings.Contains(err.Error(), "can't duplicate this flag") {
        fmt.Fprintln(os.Stderr, err, "\nNote: env/config sources also count as one occurrence")
        os.Exit(2)
    }
    return err
}

Prevention

When it happens

Trigger: Specifying the flag (or its inverse alias) twice on the command line when OnlyOnce is set; a value from Sources (env/config) combined with an explicit CLI occurrence also triggers it during PostParse.

Common situations: Shell scripts appending flags that the user also typed; alias and canonical name both present (e.g. --verbose and --no-verbose); env var mapped to the flag plus explicit CLI use after an upgrade to OnlyOnce semantics.

Related errors

AI-assisted analysis of urfave/cli@1a4deb4f5a (2026-08-31). Data as JSON: /api/errors/1c1a613d55382fe3. Report an issue: GitHub.