How to reproduce:
1. Create a new Unity Project
2. In Project -> Assets -> Create two folders with names of "Editor" and "Runtime"
3. Create a new C# script called "FruitEditor" in the "Editor" folder
4. Create a new C# script called "Fruit" in the "Runtime" folder
5. In the "Runtime" folder create a new Assembly Definition named "Unity.Fruit.Runtime"
6. In the "Editor" folder create a new Assembly Definition named "Unity.Fruit.Editor"
7. Select "Unity.Fruit.Editor" and look at Inspector window
8. Under Platforms, deselect "Any Platform", press "Deselect all", select "Editor"
9. Under Assembly Definition References, press "+", and lock the Inspector window
10. Go to "Runtime" folder and drag "Unity.Fruit.Runtime" to newly create Assembly Definition Reference
11. Press Apply at the bottom of Inspector
12. Copy script inside attached "FruitEditor.txt" to "FruitEditor" script
13. Copy script inside attached "Fruit.txt" to "Fruit" script
14. Reimport "Fruit" and then reimport "FruitEditor"
15. In Assets -> Create "Fruit"

Expected results: "Fruit" is created and no errors occur
Actual results: Error is logged in console "MonoScript is registered as both Editor and Runtime script!"

Reproducible with: 2019.4.0f1, 2019.4.15f1, 2020.1.15f1, 2020.2.0b13, 2021.1.0a7
Not reproducible with: 2018.4.29f1