** avec
"avec quelqu'un mais je ne sais pas avec qui" : 1 hit, from the  
following book about ellipsis "The syntax of silence: sluicing,  
islands, and the theory of ellipsis" (Jason Merchant)

So on Google.fr, you actually get no hits at all for that one ;-)

"avec quelqu'un mais je ne sais pas qui": 3 hits (including the same  
book about ellipsis, in which it seems that the French judgments they  
cite do not accept the sentence without repetition of the  
preposition), 2 hits on Google.fr

** à
"à quelqu'un mais je ne sais pas à qui":  5 hits

"à quelqu'un mais je ne sais pas qui": 628 hits !!!

So it seems that people accept sentences with the ellipsis of the  
preposition.



(17) A Maria dançou com alguém
the Maria danced with someone
'Maria danced with someone'
(18) Com quem que a Maria dançou t
with whom that the Maria danced
'With whom did Maria dance
(19) *Quem que a Maria dançou com t
who that the Maria danced with
'Who did Maria dance with'
(20) A Maria dançou com alguém, mas eu não lembro com quem
the Maria danced with someone but I NEG remember with who
'Maria danced with someone but I don't remember with who'
(21) A Maria dançou com alguém, mas eu não lembro quem
the Maria danced with someone but I NEG remember who
'Maria danced with someone but I don't remember who'