Tombstones remain until you fully loot them. The skull and crossbones remains wherever you died last.

If you died a second time before looting the first tombstone, the location indicator on your first tombstone disappears and shows the second one instead. However, the first tombstone is still there.

That why it's a good practice to get into to put your own marker where your tombstone is in case that happens you don't lose track of it. Hope this helps