Using track lanes: can't monitor the first while overdubbing

Hi Mario. Have you tried changing the record mode? Left click and hold on your record button in the transport module and choose Sound on Sound from the drop-down list.

If you want to do this in the same track while using Comp mode here's a workaround.
 
Record a clip for the entire time range you intend to record you new part into. You don't have to play anything. Just create a clip for the entire range. This will automatically mute the original part (the part you seemingly want to hear while you re-record). Once that is done delete the NEW take lane that was just recorded (click the X button on the Take Lane control area). Then click on the original clip (the one you want to hear) to select it then press "K" on your keyboard to unmute that clip.
 
Now when you record again that original clip will remain audible.
 
That's Comping mode though. Sound On Sound mode is probably the better option for this (Right Click on the Record button in the Transport module to open the Record Prefs and choose Sound On Sound).
 
HOWEVER... the way I do this (and the best way IMO) is to record into another track. That way you can adjust levels and tweak the pan so that you can actually HEAR the original track to play along with. If you simply record into the same track what happens is your new part will blend into the old part as you play making it hard/impossible to hear. By recording into a different track and setting the pan of each slightly opposite of each other (like maybe 10-15% or more to each side) you'll be able to hear the parts more distinctly.
 
To do this easily simply click on the original track (that has the part you want to listen to while you record) to put it "In Focus", Right Click, select "Clone Track". When the "Clone Track" dialog pops up just uncheck the box that says "Events". That will clone the entire track with all it's "Properties" (effects, volume, pan, etc) without all the clips/lanes that are in the original track (it will be empty).
 
Set your pans on both tracks and maybe turn the volume up on the new track so you can hear what you are playing better then record your part. Make sure you disable the Record Arm button and Input Echo button on the original track so you aren't listening to/recording to that track (only the new track).
 
Once you are done if you want to continue using the original track just drag the clips you recorded into the original track (hold the "Shift" key and drag the clip(s) up into the original track to keep them at the exact point in the timeline). When I do that I usually create a new, empty Take Lane to drag the clip into so there isn't any weirdness/auto lane creation.
 
Alternatively there is the Auto Punch feature as Dwardzala mentioned where you can keep the original clip audible in the same track as you record. That is more setup and again makes it hard to hear the original. If you want that go to Edit > Preferences > Project > Record (or I think Right Click on the Record button in the Transport Module). Check the Auot Punch option. Set the Auto Punch start and End times and UNCHECK the box that says "Mute Previous Take".
 
That will make it so recording will only turn on between the preset punch points AND keep your original take audible.
 
So yeah... lots of options but seriously... if you want to actually HEAR the old take with the new take doing it in a new track as I described with some panning is the best way.
 
 
Cheers.