Perhaps they are different versions, and one of the scripts depends on a later version. Having multiple links to the jQuery library is like running a new power cable into your house every time you want to add a new power point. Of course, you only need on electricity cable into your house to power the whole thing.
Make sure that the one library link sits above any other jQuery scripts. As you say, it's tough with a lot going on in the page, but it may be worth stripping the scripts out and adding them back in one by one. Then you know where the problem starts. Also, I'd recommend putting all scripts at the bottom of the page (just before the closing <body> tag). It seems to cause problems if you have the jQuery library linked at the top of the page but other jQ scripts at the bottom of the page, so I prefer to keep them all together, and preferably at the bottom, as the page visually loads faster that way (I find, anyhow).