Was Heretic a Doom knockoff?

Yes, actually. Explicitly. Video game nerd history pedantry alert: John Romero himself pitched it as a Doom-like game but in a medieval fantasy setting, and id prevented Raven Software from implementing too many of the RPG mechanics in it that they originally wanted to, instructing them not to deviate from the Doom formula. John Carmack's instructions were, "Do it like Doom, and add the fantasy flavor."

id also hired Raven to make the game in the first place, though, so it's kind of hard to be a rip-off when you have the support and permission of the creators of the original thing.