I'm building a site, adding and removing blocks all over the place many times. But what I notice is that some of my blocks which I deleted some time ago started appearing again. This is so weird! I disabled and cleared cache, deleted stacks, deleted the appearing blocks hundreds of times, but occasionally they keep popping up.

When they appear, if I go to page Edit mode and delete the block, it appears deleted, I save the page, and it pops up again. If I refresh the page twice (!), it disappears. This is really annoying and pretty bad for a real site. Has anyone experienced this? Any ideas how to fix this? How do I delete the blocks for EVER?

Thank you.