Entering an obscuring nebula causes duplicate sensor ghosts

**Describe the bug**
PvK's galaxy map shows his empire's presence in a system that was just vacated, even though his fleet left that system and entered an obscuring nebula system.

**To Reproduce**
Steps to reproduce the behavior:
1. Make a fleet
2. Move the fleet into a system adjacent to a nebula that has no other ships or colonies of yours
3. Move the fleet into the nebula
4. End your turn

**Expected behavior**
The galaxy map shows your presence in the nebula system, and not in the system you just left.

**Actual behavior**
The galaxy map shows your presence in both systems.

**Screenshots**
N/A

**Additional context**
N/A