You can apply before receiving your old one... I have been doing it from my 1st application of GCMS....
Let me tell you how GCMS works...
For example:
If you apply on 1st of November.....They will generate the file some time around the same day or two and will email you on or after 30days meaning on November 30th.
But lets say you applied a second set before 30 days
For example on 16th November you applied for GCMS.... they will generate a file around 16th November and will give you on November 30th itself.
Meaning you are recieving latest notes
I did it three times I had the same result