Lynchian characters?

[Particle_Man]

Is there any means (shape changing?) whereby a creature or character can duplicate another character so well that they forget that they are not really the character they are duplicating? Is there a pc option for that sort of thing?

I was thinking of trying a David Lynch style campaign where the party slowly realizes that they are not who they thought they were.

[Inevitability]

Programmed Amnesia is a 9th-level spell that can do what you want: a caster could hypothetically use it on themselves for Death Note-esque mind games.

A self-resetting trap of Modify Memory with a Trap Haunt to serve as its guiding intelligence might work as some kind of 'reprogramming machine', stick a Ring of Sustenance on someone, tie them up, and throw them in for a couple days? It takes a long time to overwrite someone's entire life, but you can get quite far by altering a bunch of key memories.

Taking the Second Impression skill trick and arguing to the DM that you can use it on yourself might be worth a laugh. It notes 'if you succeed, the observer ignores the evidence of his own senses in favor of what your disguise attempts to show'. You can take it as early as 3rd-level, even!

[Telonius]

If you're doing a true "replacement" scheme from the BBEG, a casting of Mindrape (from Book of Vile Darkness) followed by Programmed Amnesia would do it. Mindrape on the target to learn "everything that creature knows." Then, cast Programmed Amnesia on a "Polymorph Any Object"-ed replacement, drawn from one of your minions, to overwrite their entire personality and memories with the target's. (Winter Soldier activation optional).

(Mindrape does a similar thing as Programmed Amnesia, but has a much quicker casting time, longer range, and no material component. Unfortunately it also lacks the Programmed Trigger or anything like it).

[Gruftzwerg]

I second Mindrape. You instantly know everything your foe does and can delete, add or alter any memories and feeling (including alignment). You can even delete any memories of the Mindrape itself.