Sounds a little weird, but Conrad Murray -- who has already been convicted and serving time for killing Michael Jackson -- will exercise his 5th Amendment right against self-incrimination in Katherine Jackson's lawsuit against AEG.

Michael's mom is suing AEG, claiming the company negligently hired and supervised Murray, which resulted in her son's death.  Katherine has subpoenaed Murray to explain his arrangement with AEG.

Sources connected with the case tell TMZ ... Murray's lawyer, Valerie Wass, will file legal docs ... arguing she is appealing Murray's conviction, and if it's overturned it's likely he'd be re-tried.  If that happens, what he says could still be used against him, so Wass wants Murray to keep his trap shut.

It's pretty crazy, since Murray has already served almost a year-and-a-half.  By the time his case is appealed, he'll probably be out.