Is it just me or does Enemies Respawn at many levels, they keep coming if i don't move on to next check point in many levels.

Yeah it happens, but of course its not a big deal since the ceph have the alien tissue which you collect

The only places I have seen enemies respawn was a one-time wave of new enemies each time you blow up a munitions tent at the church level (not infinitely respawning), in the main hall of grand central station when fighting the pinger, and when fighting the pinger in Time Square. So only 3 places. And even then it's not that annoying, like COD games. I still don't see what people saw in any of those campaigns.

Could do with a good Ceph farming spot for the tissue

If I don't move enemies respawn again. It happened to me two times but I think it's not a big deal for players^^.