See https://answers.atlassian.com/questions/17004/multiple-instances-of-p4merge

p4merge ends up launching multiple instances. You can prevent this by using the 'launchp4merge' script instead of 'p4merge', but unfortunately that doesn't work when you call it directly. It appears you need to call it via another wrapper script which includes the hard-coded path. This is awkward when the location of p4merge may not be predeterminable, may need to create a wrapper script on demand in App Support.