Confused about great lengths.....?? :s

Ok, so when i had my horrible extensions put in i went to the only other salon in my area that did hair extensions for a second opinion. They happened to do great lengths extensions. Now i thought that great lengths hair was only able to be used once because of their fusion method, but when i was in the salon this lady was getting her extensions redone and apparently she had been using the same hair for 2 years (it was european hair).
So now im confused,because the old bonds that were being removed looked exactly the same as the new bonds (which were great lengths bonds).
Am i wrong in thinking that the hair cant be reused? or do you think that maybe its just the great lengths hair itself that cant be reused (possibly because their tipping method melts when they remove the old fusion bonds??)
If anyone could shed some light on this it would be much appreciated lol

Ah ha thanks for that Sherrie lol