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With Duplicati, every backup is effectively a “full” backup from a restore perspective. You can restore any file that was present at the time a backup was taken, even if that file is old. Duplicati is also an “incremental forever” type of backup in that it only processes new and changed files each backup job.
These terms carry a lot of baggage and it can cause confusion because backup products use the terms differently. Rest assured, there is no problem with deleting the first backup because with Duplicati there is no dependency chain among backup versions.
See this thread where it was discussed recently.