He's back.
2 seconds sound clip from Darkwing Duck - Just Us Justice Ducks Part 2 (S1E21).
You can hear this line at 00:16:05.872 in the DVD version of the TV Series.
Quote context
[...]
- Kill NegaDuck. Kill NegaDuck.
- No, no, no. Look, I'm Darkwing Duck. He's NegaDuck. Kill him.
- You thought he was me? You were going to kill me?
- No, no. We knew it was Darkwing.
- Yeah. We were just testing, see?
- Didn't we already kill Darkwing Duck?
- Yaah! The ghost of Darkwing Duck.
- The Justice Ducks will triumph terrifically, you treacherous...
- Note the critical condition of your captured compatriots.
[...]