This is a very interesting topic and could well become a large and never-ending debate.
A set design is more than a colourway. It's specific colours used on specific keycaps with a specific profile per keycap, specific legends per cap, etc, and as such constitutes a "design". This set of choices is recognisable as a complete design and therefore is copyrightable. For instance, if I say "Hack'd by Geeks", people recognise and know that it's a sculpted SA profile set in 3-1-2-3-4 with Gorton Modified font, in GX, GE and OAS colours from Signature Plastics ABS colour range. It's a "thing" in the same way as a painting made by selecting particular colours and arranging them in particular positions on a canvas is a "thing", although it requires less skill to create and is more limited in the possible positions of colours and the actual colours that can be used.
However, it's not altogether relevant. In the real world it's all about how the designers and manufacturers use them. Allow me to elaborate:
SP recognises set designs as "designs". They separate and identify them that way. They require designers to sign documents licensing the design, etc. In this sense they recognise the designer's rights to a design, but they don't control the possibility of copying / plagiarism, they simply require that the designer is not breaching copyright with their design. This is up to the designer. If there is an actually registered copyrighted / trademarked element that they want to use in their design they are required to get a letter of permission from the copyright / trademark owner to create a derivative work. As long as this criterion is covered, they will make any design. An example is what's happened in 00Zero's Out of the Vault series 3. The Nuka Cola legend required a letter of permission which was refused... GB busted.
I am creating a new font to be used with SA keycaps. This font will not be copyrighted. It's a free and open thing, but SP will consider me the "owner" of the font if I submit and pay for the whole project. I plan to make all the contributors co-owners, at least in terms of control rights (any person who contributed to the cost can give permission for it's use). This is how SP does things. It doesn't stop some Chinese company copying the font and making keycaps with it, but if a designer uses it in their design and submits it to SP, SP will require permission for it to be used. I want to make this as open as possible, so it will be an inclusive agreement (still need to confirm how to do this in a legal way, but it shouldn't be too difficult), which means it requires only one person to give permission, even if some others refuse permission. This is to make it so one or two disgruntled "owners" can't prevent it being used by the community, as that is what we're creating it for.
In terms of designs and plagiarism, usually at the interest check stage, someone will mention any similar designs. Depending on the design similarity and the designers themselves, they will usually come to some sort of arrangement where nobody steps on the other's toes. This is a community-level process / discussion, no legalities involved. An example is what happened in the Classic Space IC. One of the original renders was very similar to Cospar / Godspeed. This similarity was noticed, mentioned and dealt with. Now we have two different designs, both moving forward, and designers (all three) happy with the designs and differences.