Prevent reference before assignment error

[rtibbles]

Description

• If a node with the specified id already exists in the backend when we try to duplicate, this can lead to an IntegrityError
• We then fail to have a referent for passing the change back to the frontend
• This case likely happens when a copy change is sent to the backend twice

Issue Addressed (if applicable)

Fixes#2904

[codecov]

Codecov Report

Merging #2981 (435f144) into hotfixes (28587f8) will increase coverage by 4.94%.
The diff coverage is 92.92%.

@@ Coverage Diff @@## hotfixes #2981 +/- ##
============================================
+ Coverage 80.80% 85.74% +4.94% 
============================================
Files 281 298 +17 Lines 12659 15845 +3186 ============================================
+ Hits 10229 13587 +3358 + Misses 2430 2258 -172 

Impacted Files	Coverage Δ
contentcuration/contentcuration/decorators.py	56.60% <50.00%> (-37.60%)	⬇️
...ntentcuration/contentcuration/db/models/manager.py	91.20% <90.98%> (-8.80%)	⬇️
contentcuration/contentcuration/forms.py	82.35% <94.04%> (+33.89%)	⬆️
contentcuration/contentcuration/api.py	92.06% <100.00%> (+1.43%)	⬆️
...tentcuration/contentcuration/context_processors.py	100.00% <100.00%> (ø)
...ontentcuration/contentcuration/db/advisory_lock.py	100.00% <100.00%> (ø)
...tcuration/contentcuration/db/models/expressions.py	93.33% <100.00%> (-6.67%)	⬇️
...entcuration/contentcuration/db/models/functions.py	100.00% <100.00%> (ø)
...ation/contentcuration/tests/test_rest_framework.py	36.93% <0.00%> (-63.07%)	⬇️
contentcuration/contentcuration/utils/format.py	21.05% <0.00%> (-49.54%)	⬇️
... and 181 more

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[rtibbles]

Holding off merging until #2976 has been merged.