ll (double l) convertion error

I have been using Calibre on my Ubuntu laptop for some time and done conversion of pdf documents to MOBI format (Kindle).

My problem is that the conversion goes wrong with words with double l. see examples below:

carefully careful y
wall wal
tall tal
really real y etc etc

It is a funny but irritation error but I havn't been able to find a solution to the problem.
I have just joined the Forum hoping to get a solution.

I havn't found any other errors in the conversion.