Description of problem:

If a user adds a product to a system template and then also adds to the template one of the repositories that exists in that product, the generated TDL contains multiple entries for that repository.

This bugzilla is to ensure that when TDL export is generated, the output does not include duplicate repositories.

Version-Release number of selected component (if applicable):
git master

How reproducible:
always

Steps to Reproduce:
1. Create a product/provider/repo
2. Sync the repo
3. Go to Content Management -> System Templates
4. Create a new system template
5. Add the product from 1 to the template
6. Add the repo from 1 to the template
7. Save the template
8. Download the template
  
Actual results:
Observe that the template exported (downloaded) contains duplicate entries for the repo.

E.g. 
<repository name="rhel 6-1 repo">
<url>
https://localhost/pulp/repos/ACME_Corporation/Library/custom/rhel_6-1/rhel_6-1_repo
</url>
<persisted>No</persisted>
</repository>

<repository name="rhel 6-1 repo">
<url>
https://localhost/pulp/repos/ACME_Corporation/Library/custom/rhel_6-1/rhel_6-1_repo
</url>
<persisted>No</persisted>
</repository>

Expected results:
The repo should only be listed once in the TDL exported.

Additional info: