Copy the Copies, Backup Your Backup

It takes many copies of files stored on separate pieces of media to fully ensure that prior work can be recovered.
Redundancy is security. In order to achieve it, use multiple pieces of media. Multiple copies of data on a single piece of media still leave that data at risk. Data security is achieved only through media redundancy on reliable media.

Duplicating all full backups, including weekly, is ideal since this is generally the only existing full record of everything that has happened. The loss of a weekly full backup just prior to the next one means that your most recent complete record of all files is two weeks old.

Duplication is especially critical for those tapes that have longer retention periods. Therefore, at a minimum, monthly, quarterly and yearly tapes — all or some of which may be archive tapes — should be duplicated, since the loss of a single copy of one of those tapes can have a significant impact on the business. (See archive section for more information on archiving.)

Finally, at least two copies of any long-term archive tapes should be placed in separate vault sites for maximum protection. A third duplicate can be made and kept onsite for quick restores.
Duplicate vs. Cloning
 
The best possible form of duplicating your data is to create multiple copies in real time, as the backup is occurring. This is the highest level of data protection. Duplication can also be done at the completion of a backup (this is sometimes called Cloning). It may even occur the following day. This is usually when there are not enough tape drives available for online duplication. Though less costly, it is also less ideal, as there is a window of time when only one copy of the data is available. Therefore, if practical, it is better to create duplicates during the backup. However, creating duplicates anytime is preferable to not creating them at all.