<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />Potential Workarround/Solution:
To address this issue you first have to check the version of JMP and of the OS. Here are one discussion thread in the public community as well a support thread talking about a similar issue:
Re: why does JMP 10 create a new instance when opening a data table
52261 - New instance of JMP® is opened each time a new file is opened
In general you should either try one of the following:
* First check the default application in Explorer matches the JMP instnace you want to open the files, and that there is just a single version of JMP installed (check if the new instance still is the same JMP version through Help->About JMP). If there are multiple the last installed will be opened unless you specify it yourself with "Open with".
* Then Reset associations in JMP Preferences under File-> Preferences, then Windows Specific

This will reset the registry entries setting the right associations to that JMP instance. Restart JMP and see what happens.
* If this will not change things to a better, the best workarround is to drag and drop the files to be opened from the explorer to any JMP Window, or use the File -> Open Option.
This issue should not occur any longer in JMP 12.
Hope this is helpful,
Martin