Does the license text of an Open Source license have to be printed again in full within the framework of own license terms, if it has already been delivered to the customer as file, e.g. as part of the software package?

No, this is not necessary. The requirements of all relevant Open Source licenses are met if a copy of the license text is delivered to the customer together with the software. The own license terms for possible proprietary components do not have to contain the license texts again, but at the same time must not be in contradiction with them (see FAQ "What has to be noted in the case of license terms for own developments if distributed in combination with free software?")