title: C# scripts in Unity opening new instance of VS15 instead of adding to existing instance
body: <p>I recently had to reinstall Unity + Visual Studio. And previously both were working fine (I had installed VS15 + Unity separately - however this time I installed VS15 through the 'Unity Tools installation')</p>
  
  <p>However since then, when I try to open a script from Unity, it opens a new instance of VS15 each time - the kicker is when it opens this new instance, it also opens the previously opened scripts too. </p>
  
  <p>So for all intents and purposes it opens all the scripts I want in one particular instance.....however to do this it for some reason opens a brand new instance rather than simply adding to an existing one?</p>
  
  <p>For example;</p>
  
  <p>I open <code>Script_A</code> by double clicking it in Unity. 
   It Opens the first instance of Visual Studio 15 (as it should - this shall be referred to as "First Instance")</p>
  
  <p>I then want to open up <code>Script_B</code> by double clicking it in Unity also. However rather than simply opening this script in the "First Instance". It will relaunch a new instance of VS15 (AKA "Second Instance"). Once the "Second Instance" has loaded it will have <code>Script_B</code> opened as well as <code>Script_A</code>. Whereas "First Instance" still only has <code>Script_A</code> open.</p>
  
  <p>The same thing happens if I open up a third, forth etc script, it will launch a new instance each time, rather htan just adding to an already opened instance.</p>
  
  <p>Is this an issue encountered when VS15 is installed through Unity launcher, or is it some other issue (preferences, settings, either with Unity or Vs15 itself?) as it is quite annoying.</p>