2022-June-15 - Prerelease - 2.6.906

Fix "Duplicate key" error due to AI code. (#10655)

Fixes #10653.

The cause was the AI code was passing overlapping attackers and defenders lists to the battle calculator.
Adds a precondition check to catch this type of error more explicitly in the future.
Also removes a stale comment that influences some class that doesn't exist.

Tested with an all-AI game on Global 40 house rules.