Update non-free.md (#574)

Add Duet under Project Management.
9 years ago · bbc30056c9
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 ## Project Management
  * [Active Collab](https://www.activecollab.com/) - Project management  - `⊘ Proprietary` `PHP`
+  * [Duet](https://duetapp.com/) `⊘ Proprietary`  - Invoicing and project management with an integrated client portal   ([Demo](https://duetapp.com/demo/#dashboard)) `PHP`

 ## Self-hosting Solutions
  * [Axigen](https://www.axigen.com/mail-server/free/) - `⊘ Proprietary` Great alternative to open source. It's a turnkey messaging solution, perfect for small & micro businesses, integration projects or test environments.