In determining that, it depends on how well bonded the existing linoleum aka sheet vinyl is down? and what type of flooring You plan to use on top of it? I primarily install ceramic tile, an I've come across several layers of vinyl on a lot of tear outs I've done. a lot of vinyl is installed with a perimeter glue down. where the vinyl is only glued on the edges. that I would recommend removing its fairly easy an must be done if installing ceramic. but if its completely glued down and is gonna be a complete bear to get up, You can go over it with a multi-purpose thinset which has a latex additive if doing ceramic. If You are using vinyl I believe the only concern is if Your original vinyl has deep grooves or patterning, it could possibly show through depending on the new product your installing. Hope this helps
p.s. if You do choose to go over anticipate a little longer drying time as the moisture cant escape the setting material as quickly do to the vinyl.