From Hull AWE
The adjective from the verb 'to debate' is spelled with only two '-e-'s, one as second letter, the other at the end - debatable. Do not spell it
debateable (although OED records this as current in the 17th to 19th centuries).
- Historically, the debatable lands were those territories on the border between England and Scotland (largely in the west) which were disputed between the two kingdoms until, and after, the Union of the Crowns.