webdiplomacy.net

Hi,
I tried attacking my own fleet in a game with a supported attack (resulted in a 2v1 situation) and my losing fleet did not have to retreat, it is like I bumped into myself. My goal was to retreat the losing fleet in an enemy empty territory, which was certain to happen due to the current unit position.

I wondered if it is a bug or if I misunderstood that you could attack your own armies/fleet

You cannot dislodge your own unit. The adjudication system (at least in classic) is very robust and has no bugs.

Here's a list of different adjudication scenarios for future reference:
https://webdiplomacy.net/datc.php

specifically, it's test case 6.D.11

I've you're really keen there's a set of every rulebook that came with the game going back to the Realpolitik days in the 50s at the Diplomacy Archive.

http://www.diplomacy-archive.com/diplomacy_rules.htm

One day I'll get round to reading one of them

There's a newer rulebook available online (easy to search and find) that is the 5th edition from 2008. In keeping with a tradition of bloat, it is an even larger pdf file.