I concur with Mark. This happens to me after creating a flightplan with FSC9 using sids and stars.

I enter the FSC9 flightplan (converted to .pln) via the flightplan page of the CDU, this adds the flightplan with the sid and star. I then manually enter the sid and star via the dep/arv tab on the CDU. This duplicates the sid and star so I have to manually remove the discontinuities and duplicate way points from the CDU.

I should do it Mark's way but it would mean creating 2 flightplans, one is .fsc and one in .pln (without sid/star) as I like to follow the dep/arv in FSC9. Pure laziness on my part lol.

To answer your question easily, load a flightplan into the CS757 CDU without the sid and star, and load then manually via the dep/arv tab on the CDU.