Outlook has a know issue of opening multiple instances in windows 7 if multiple click on outlook taskbar icon. (provided hide when minimized opted)
Here is an work around.
Step 1:
Create/replace outlook short cut in
"C:\Users\
[name]\AppData\Roaming\Microsoft\Internet Explorer\Quick Launch\User Pinned\TaskBar"
Step 2:
- Browse to locate outlook.exe
- add "/recyle" switch/option in short cut wizard
- Click next and finsh

You are good to go.