OS. [tʰn] became [ttʰ]; [tʰn] > [ttʰ]
Based on a single example, it seems [tʰn] > [ttʰ] in Old Noldorin, as described by David Salo (GS/§4.49):
It is unclear whether this sound change applied to combinations of [n] with other aspirates, as was the case when [m] became [w] after aspirates. It is also unclear whether this change could occur in later Sindarin also.