Imu knows about both DFs, Luffy, and Joyboy.
And Nidhoggr obviously, to recognize it through the eyes of Gunko immediately.
Imu has met only Joy Boy. He recognized Nidhogg as anyone with knowledge would, but there was no War God during the second world.
First:
Shirahoshi is menaced by the killer of her mother. Corazon uses his title as a cover but is caught and beaten by Vergo, then Law uses his title as a cover only to get caught and beaten by Vergo. But sure, those aren't specifically similar themes to Imu.
The king of Dressrosa was Rebecca's grandfather, who was parasited. Kyros forms a thematic trio of puppets under Doflamingo.
Judge is lured in and betrayed Big Mom's promise. Sanji is lured in and betrayed by Big Mom's promise.
Sukiyaki was deceived by Orochi, Oden was... deceived by Orochi.
So you can't even say they won't get manipulated because someone else was.
Those are all different cases.
Sanji, for example, didn't get deceived or lured: he went there willingly and so on.
Shirahoshi didn't fall as her mother, and so on.
Loki, as these other characters, was meant to succeed where the others had failed.
Anyway It's not really relevant to discuss the reasons behind why something shouldn't happen when that thing already didn't happen.