Gregory:
regarding how to delete them, zelzvga has given you the right answer. Have you tried this? Do you need any further explanation?
regarding how it happened: there was a mistake. It's hard to know exactly what the mistake was - I can give you an example of a mistake that could cause this. This is not to say that you made exactly this mistake, you could have made a different mistake. But the one I will tell you about might help you understand how synch works
Do you have access to cloud sync? If you do, the best way to migrate to a new laptop would be to install mlo on the new machine, then create a new empty MLO file. Then, while it is still empty, set it up to synch with the same cloud file that you have been using. MLO will add the two files together, your existing file with all of your tasks plus your new (empty) file, then send everything from the cloud file out to your new laptop. This is how it should work.
Suppose that what you did was to copy your old mlo file to your new computer, maybe as a part of transferring your entire hard drive over. As far as MLO is concerned, this will be a new file filled with new tasks. When you synch it to the cloud file, MLO will add your existing cloud file (containing all of your tasks) with the new file on your new laptop (which also contains all of your tasks) - at the end of that you will have two copies of each task, which MLO will send from the cloud file out to your new laptop.
-Dwight