Kind of - you may only ever submit a single URL to a single site once. Additional submissions do NOT create further backlinks, they just count as a vote for the original submission. So if you attempt to create a new job from a URL you have already used then you will get 'Duplicate URL' error message.
However, we add new sites all the time and as long as you leave your jobs in the system you can re-submit them to new sites on an on-going basis to pick up a constant stream of fresh backlinks. On the jobs list you'll see a column marked 'SA' - this stands for 'Submissions Available' and it is the number of sites that you can select for this job. Click on the ID of a job to go into the details screen, select a new batch of sites and hit the Re-Submit button. A new submission schedule will be created for these additional sites. You can re-submit in this way whenever you like.