1) If someone due to extreme waswaasa ( doubt) repeat any word in salat is his salat void (batel)?
If he doubts that word he recited was not in proper makhraj

2) And what if he tries to neglect doubt but bymistake as due to his previous addiction to doubt repeats that word ?

1- He is permitted to repeat the word.
This is wasswass, from satan.
2- No problem, but he should fight it.