I have a strings track down and it plays OK.
I want to copy the four blocks of notes and paste it on the end so it repeats.
I do a lasso and copy all the notes and then paste, but the Duration of the first block (highlighted in dark purple below) is wrong.  It just keeps on playing and does not stop and this obviously makes it a muddy sound because I've then got two chords playing at once.

 
On the copy of the MIDI notes, the second, third and fourth block (in lighter purple above) play OK and end when they are supposed to end. 
Its just the first block that keeps on going.  When I right click and check the "duration", the copy is the same as the original.  Its just that the copy actually plays longer than the original.
(When I copy I do copy all the events/key changes/markers etc).
Any ideas?
 

So it's not the visible durations that are wrong but you don't get a note off event yes?
 
Try this - switch global Snap OFF ('n' to toggle on/off)
Select all the notes of the first chord in the prv and slip edit the right hand edge back just a few ticks, so they don't abut right up to the start of the notes in chord 2
 
Try it.

Check the Event List or Controller Pane in PRV for Sustain (CC64) controllers.

Thanks so much!   I did find an extra sustain in there.  Although how it gets there I have no idea.