Hello @Inolas07 and welcome to the community.
No letter can be used more than once in any scale.
There are seven letters only which can be used, running from A to G inclusive.
Where a note within a scale would ordinarily take one name, if that letter has already been used, then it needs to be renamed using an enharmonic equivalent and flats / sharps.
The E# must be used.
E# is the same as F but the letter F has already been used.
You can not use the same letter more than once.
You must use all seven letters once each.
f E# was named as F, then it was followed by F#, the letter F is used twice and the ltter E is not used at all.
That breaches the rule above.
Cb is an enharmonic equivalent to B but the letter B has already been used. The same as above.
Cb major = Cb, Db, Eb, Fb, Gb, Ab, Bb
I hope that helps
Cheers 
| Richard_close2u | JustinGuitar Official Guide, Approved Teacher & Moderator