fix: reuse existing row on dual write if available

[eschutho]

SUMMARY

We're seeing some integrity errors

psycopg2.errors.UniqueViolation: duplicate key value violates unique constraint "sl_columns_pkey"
DETAIL:  Key (id)=(x) already exists.

on import.
To avoid a duplicate primary key violation, I'm copying the existing row from the database instead of copying just the id and trying to use Sqlalchemy to merge the data since the add/merge functionality isn't in this module.

TESTING INSTRUCTIONS

This is an intermittent error and difficult to reproduce but happens for some people on import.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #20661 (84910b6) into master (b39a3d8) will increase coverage by 0.03%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master   #20661      +/-   ##
==========================================
+ Coverage   66.82%   66.86%   +0.03%     
==========================================
  Files        1752     1752              
  Lines       65616    65574      -42     
  Branches     6938     6938              
==========================================
- Hits        43849    43845       -4     
+ Misses      20007    19969      -38     
  Partials     1760     1760              

Flag	Coverage Δ
hive	53.87% <62.50%> (+0.05%)	⬆️
mysql	82.47% <87.50%> (+0.08%)	⬆️
postgres	82.54% <87.50%> (+0.08%)	⬆️
presto	53.73% <62.50%> (+0.05%)	⬆️
python	82.98% <87.50%> (+0.09%)	⬆️
sqlite	82.35% <87.50%> (+0.11%)	⬆️
unit	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/connectors/sqla/models.py	90.33% <87.50%> (+0.01%)	⬆️
superset/initialization/__init__.py	89.93% <0.00%> (-1.74%)	⬇️
superset/databases/commands/validate.py	74.13% <0.00%> (-1.67%)	⬇️
superset/utils/log.py	93.70% <0.00%> (-1.40%)	⬇️
superset/views/base.py	75.36% <0.00%> (-1.36%)	⬇️
...t/annotation_layers/annotations/commands/delete.py	84.00% <0.00%> (-1.19%)	⬇️
superset/dashboards/permalink/commands/get.py	81.25% <0.00%> (-1.11%)	⬇️
superset/annotation_layers/commands/bulk_delete.py	84.61% <0.00%> (-1.10%)	⬇️
superset/css_templates/commands/bulk_delete.py	87.50% <0.00%> (-0.97%)	⬇️
...rset/queries/saved_queries/commands/bulk_delete.py	87.50% <0.00%> (-0.97%)	⬇️
... and 91 more

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[betodealmeida]

Seems reasonable... I hope we can finalize the migration and get rid of this code soon-ish.