I've had this BB 8900 for about 2 month, awesome phone but since about a week the trackball started this odd behavior: some times when the trackball is pressed it will act as if it is pressed twice, for example, if I want to go to messages, it will not only go into messages but will open the top messsage. If I click on a message it will open the message and the menu at once.

This issue is more apparent when pressed a bit harder than usual.

Please advice.

Two thing's I would do.

One.
Since it is somewhat new take it to the nearest provider store and have them take a look at it. With any luck they will replace it.
NOTE: IF you have a warranty USE IT.

Two.
If you're comfortable with taking gadget's apart, I'll give you a link and some instructions on taking your phone apart and maybe giving the trackball a good cleaning.
(This may not be the problem, but if I were to take a guess its the contact under the trackball either has something on it, or it's just not making a proper contact.)
NOTE: This void's any warranty's you have on the phone, so if you have a warranty, USE IT.

Then again, this just might be your problem, period. hahaha

Took it to the provider, they tried to repair it by cleaning the trackball area and changing the trackball itself but the issue continues ... it is really annoying as you end up sending e-mails without content! ... double clicking by itself is a bummer ...

Yeah, I'm going to say it's the contact underneath the trackball that's having a problem.
(What is actually "clicked" when you press the ball down"
There's ways of cleaning it, but replacing it not so much, you're better off asking them to replace the faulty phone they gave you.