Unique Values in Data Validation List

Hello all,

I have put together a sample spreadsheet of an issue I am experiencing, but the main goal is to have 2 drop down lists using data validation to only show unique values:

Box 1 - Project 1 or 2. The range is pulling from a range that includes duplicates, so I only want to show these 2 options
Box 2 - Milestone dates as a drop down list, but I want the list to be flexible depending on the value selected in box #1.

*spreadsheet attached*

Thanks for your time on this!

Let me know if you have questions
SB