Use `subsume` when checking for repeated varnames

[torfjelde]

@sunxd3 pointed out that we should be using subsume rather than equality when checking whether a varname has been seen in the model already (https://github.com/TuringLang/Turing.jl/pull/2218#issuecomment-2100722690).

This PR does exactly that.

[coveralls]

Pull Request Test Coverage Report for Build 9021828428

Details

• 5 of 15 (33.33%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 78.519%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/debug_utils.jl	5	15	33.33%
<!--	Total:	5	15	33.33%	-->

Totals
Change from base Build 8991588253:	-0.2%
Covered Lines:	2789
Relevant Lines:	3552

---

💛 - Coveralls

[torfjelde]

@sunxd3 @devmotion @yebai can one of you have a look at this? Should be a quick merge

[sunxd3]

@torfjelde do you think is it worth it to add test for member test (e.g. a and a.b)? Otherwise, look good to me. Thanks for this.