Generation of random manual takes longer when the max number is being reached

[MKruger777]

When deciding which will be the next manual to be sent, the completed file (containing the ids of the manuals already sent) is being checked.
If the random exists in the completed file, a new random number is generated.
This continues until a number is found that does NOT already exist in the completed file.

This generation of random manual takes longer as the number range approaches its end/max

The symptom is that the process starts to become slow

[MKruger777]

The solution probably lies with the logic that determines the next manual id to be sent. This process should be the target of the fix.