When a script is opened using MonoDevelop (double click in Asset tab on a script), every time a new instance of MonoDevelop is created. As a result, it can open unlimited MonoDevelops and eat memory.

Steps:
1. Create a new project
2. Create a new script
3. Double click multiple times on a script (to open in MonoDevelop)

Result: multiple MonoDevelops are opened.

Regression introduced in: 2017.3.0a7.
Not reproduced with: 2017.3.0a6, 2017.2.0p1, 2017.1,2.p2, 5.6.4p1.
Reproduced with: 2017.3.0b7, 2018.1.0a2.