Can Data Uploader be used to fill in 4-5 columns of data from a Legend Table, where the Unique Identifier Column could be duplicated in data set?
Yes, it sounds like the reason it's skipping all the lines is because the Unique Identifier isn't actually unique, so Data Uploader can only upload information for the first instance of that attribute.
In this scenario I would normally suggest using cross-sheet formula, which would find the matching attribute and populate the columns based on this, as you noted at the end of your post.
If you're running into limitations with a VLOOKUP, try using an INDEX(MATCH formula instead, which would just look at the two separate columns instead of a vlookup range. This helps limit the number of cells the formula has to look through to find the match. You can find examples of this type of formula in the Community, like in this post here.
Additionally, instead of Data Uploader you may want to look into Data Mesh (see bullet point number three in this Help Center article, here.) This would still require your legend to be within Smartsheet, though.
I hope that helps!
I'm not sure I understand the second half of your question, "where the Unique Identifier Column could be duplicated", would you mind expanding further?
For the first half of your question, yes! Data uploader will match up the Unique Identifier Column and then you can definitely map 4 -5 columns to be updated based on the content in those matching columns from the source data (see this Help Article for more information).
Now, if you had duplicated content in that Unique Identifier Column (ex. two rows titled "Test"), Data Uploader will only match up and update the 5 columns/cells that are associated with the first match, so your second row that says "Test" would be blank in those columns. Is that what you meant?
I am bringing in a listing of sites and several of these sites have the same attribute that I am trying to key off of to apply data from a source sheet. For example there are 500 sites and of those sites there are 400 sites that share one of 20 attributes. I have a legend that aligns to each attribute that I am trying to bring that data into the sheet via data uploader. We I select the Unique Identifier column in Data Uploader and run the report is simply skips all lines. Is it because the items in the Unique Identifier column is duplicated throughout the data?
I am trying to do this versus doing a vlookup due to the limitations. I got the vlookup to work but I had to change my legend around which creates more sheets to deal with.