How to reproduce:
1. Open the "Main" scene in the attached "1046004_repro.zip" project
2. Create a custom Tile Asset (Asset/Create/Tiles/TEST)
3. Select the newly created Tile Asset and assign its' "Sprite" field a value
4. Drag and drop the Tile Asset into the Tile Palette
Optional steps:
5. Select that Tile from the Tile Palette
6. Place it in the Tilemap (in the Scene)
7. Move the cursor from the placed Tile

Expected result: When placing the Tile Asset into the Tile Palette or placing a Tile into the scene, the StartUp method for that Tile gets called as many times as there are Tiles placed.
Actual result: When placing the Tile Asset into the Tile Palette or placing a Tile into the scene, the StartUp method for that Tile gets called more times than the number of Tiles actually placed (view debug messages in the Console).

Reproducible with - 2017.2.3p1, 2017.4.5f1, 2018.1.4f1, 2018.2.0b7, 2018.3.0a1

Notes:
- The issue does not occur in the Play mode (when entering the Play mode, the StartUp method is called once for each Tile in the Scene and in the Tile Palette)
- Could not test on 2017.1, since Test Palette was introduced in 2017.2.