Indeed, added items will not be deleted. This could lead to unexpected situations e.g. for saved tables: existing items will be overwritten, not present items in the backup will still be there and other strange unexpected scenarios could occur. So restoring a DMA must be done on a server that is clean and ready for installation.
Please check the DataMiner Help chapter https://community.dataminer.services/documentation/preparing_the_destination_server_for_a_dma_restoration/ where it's explaining to restore a DMA one should either install a blank, fully functional DataMiner Agent, or clear the configuration of an existing DMA. The DeleteAllDataDMA.dmupgrade package, that can be found under the Tools folder, can also be used to clear the configuration.