Are the extra A's in the imperfect an error?
Not if OCS evidence is to be believed.
Just wondering. They do look like errors, though. Nowhere else do you find double A's in Slavic languages. Is it possible they were pronounced -aja-?
This is one of the great mysteries of Slavic linguistics...
So is OCS the only language they are attested in?
Maybe East Slavic too, I don't know. The modern Bulgarian imperfect has just a single vowel.