(core) - Add changeset for #1515

.changeset/quiet-tips-drive.md

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+---
+'@urql/core': minor
+---
+
+Add consistent "single-source behavior" which makes the `Client` more forgiving when duplicate
+sources are used, which previously made it difficult to use the same operation across an app
+together with `cache-and-network`; This was a rare use-case, and it isn't recommended to overfetch
+data across an app, however, the new `Client` implementation of shared sources ensures that when an
+operation is active that the `Client` distributes the last known result for the active operation to
+any new usages of it (which is called “replaying stale results”)
+
+See: [#1515](https://github.com/FormidableLabs/urql/pull/1515)
+