For some builds of NAV there seems to have this issue when you run an object from development environment, it opens new instance of windows client for each run. This is annoying.
To fix this run the following command in command prompt as administrator.
C:\Windows\Microsoft.NET\Framework\v4.0.30319\RegAsm.exe /register "C:\Program Files (x86)\Microsoft Dynamics NAV\90\RoleTailored Client\Microsoft.Dynamics.Nav.Client.WinForms.dll" /tlb
PS: the above command is for NAV 2016. So you will have to give the correct path based on your installation for this to work.
h/t to this post from mibuso