fix(reactivity): watchEffect should deduplicate multiple onCleanup calls for the same function

[shengxj1]

fix #3341

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Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	99.8 kB (-239 B)	37.9 kB (-59 B)	34.2 kB (-48 B)
vue.global.prod.js	158 kB (-239 B)	57.7 kB (-61 B)	51.3 kB (-90 B)

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	47.1 kB (-103 B)	18.3 kB (-18 B)	16.8 kB (+34 B)
createApp	55.1 kB (-90 B)	21.3 kB (-19 B)	19.5 kB (-26 B)
createSSRApp	59.1 kB (-152 B)	23 kB (-28 B)	20.9 kB (-39 B)
defineCustomElement	60 kB (-88 B)	22.9 kB (-22 B)	20.8 kB (-14 B)
overall	68.9 kB (-193 B)	26.4 kB (-61 B)	24 kB (-122 B)

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commit: 6a13a05

[skirtles-code]

I'm unclear what the motivation is behind this change. Why do we need to deduplicate?

If the callback function is stateful (e.g. calling pop() on a stack) then calling it multiple times might be important. I don't know how realistic that kind of use case is in practice.