chore: deduplicate subsequent identify calls

[joshsny]

Often duplicate $identify calls are made due to the SDK being incorrectly setup in the app, or due to re-rendering of providers.

The current behaviour is to only deduplicate $identify calls with no properties, but most $identify calls include properties like email, so they are not deduplicated. This PR extends deduplication to calls with properties.

Note: this is my first contribution to js sdk so would appreciate a thorough review.
...

Checklist

• Tests for new code (see advice on the tests we use)
• Accounted for the impact of any changes across different browsers
• [x ] Accounted for backwards compatibility of any changes (no breaking changes in posthog-js!)

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	Jan 14, 2025 6:04pm

[github-actions]

Size Change: +3.46 kB (+0.11%)

Total Size: 3.24 MB

Filename	Size	Change
dist/array.full.es5.js	264 kB	+350 B (+0.13%)
dist/array.full.js	367 kB	+346 B (+0.09%)
dist/array.full.no-external.js	365 kB	+346 B (+0.09%)
dist/array.js	180 kB	+345 B (+0.19%)
dist/array.no-external.js	179 kB	+345 B (+0.19%)
dist/main.js	181 kB	+345 B (+0.19%)
dist/module.full.js	367 kB	+346 B (+0.09%)
dist/module.full.no-external.js	365 kB	+346 B (+0.09%)
dist/module.js	180 kB	+345 B (+0.19%)
dist/module.no-external.js	179 kB	+345 B (+0.19%)

ℹ️ View Unchanged

Filename	Size
dist/all-external-dependencies.js	207 kB
dist/customizations.full.js	12.6 kB
dist/dead-clicks-autocapture.js	14.4 kB
dist/exception-autocapture.js	9.48 kB
dist/external-scripts-loader.js	2.64 kB
dist/recorder-v2.js	115 kB
dist/recorder.js	115 kB
dist/surveys-preview.js	58.1 kB
dist/surveys.js	63.8 kB
dist/tracing-headers.js	1.76 kB
dist/web-vitals.js	10.4 kB

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[pauldambra]

brain dumped from a quick scan... i've not really worked with identify so haven't thought about whether we should 🙈

[pauldambra]

was thinking about this...
i don't think it's breaking (even though it's technically breaking) because this is the behaviour that I think humans expect, so I don't think we need the hit on bundle size of adding config to opt in and slow rollout and all that jazz.

we should label as minor change though (that's what does CI here)

I didn't approve because I don't know why we shouldn't do it - which is too many double negatives - let's update docs on posthog.com and get @joethreepwood to put it in changelog

[benjackwhite]

Personally I think this makes total sense. I can't think of a reason why it wouldn't want to be this way, plus we have a warning logged for it so good from me