how come in season 7 the first can take the form of Buffy?

The First is able to take on the form of anyone who has died (Spike, and Dru are both 'dead'). Buffy has died twice (the first death in the puddle business offically ruled as a real death because that is the reason Kendra was able to be Called) so the First is able to take on her Form.

The other big bit about the First is in Potential (I believe), in the beginning of the season Giles is shown with his head about to be sliced and diced and we cut to black and new get a resolution and then (if anda watch very carefully) nobody makes physical contact with Giles until that episode - no hugs, no handshakes - so it was thought that Giles could be dead and the First was taking on his form.

To get back to the question. Buffy died, sure she was brought back but she was dead (just like Spike and Dru) so the First can take her form.

because she is dead.she had died 2 times!!!