Are all GTINs unique numbers?

Yes, if they are correctly created from a GS1 company prefix or assigned directly by a GS1 Member Organisation. Whether they are 8, 12, 13 or 14 digits in length, they cannot be confused with one another.

For applications that require a uniform 14-digit format, leading zeroes need to be added. See https://www.gs1.org/standards/id-keys/gtin for more information.