Luke 12:29 has the difficult phrase, "Neither be ye of doubtful mind." Therefore take no thought ( μὴ οὖν μεριμνήσητε). The shade of difference here and Luke 12:34 from Luke 12:25 cannot be expressed in an English translation.
In Luke 12:25 a state of anxiety, here and Luke 12:34 : one anxious thought, is forbidden.