Add documentation and examples for GHC-59692: Duplicate Instances

message-index/messages/GHC-59692/index.md

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+---
+title: Duplicate Instances
+summary: Multiple instances defined for the same type class and type.
+severity: error
+introduced: 9.6.1
+---
+
+For type class coherence, at most one instance may be defined for each type for the same type class.
+
+Identical instances should simply be removed.
+If multiple instances for the same type are required, circumventing this restriction is possible by introducing a `newtype` wrapper.

message-index/messages/GHC-59692/multipleInstances/after/MultipleInstances.hs

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+module MultipleInstances where
+
+newtype Sum = Sum { getSum :: Int }
+
+instance Semigroup Sum where
+    (<>) x y = Sum (getSum x + getSum y)
+
+newtype Product = Product { getProduct :: Int }
+
+instance Semigroup Product where
+    (<>) x y = Product (getProduct x * getProduct y)

message-index/messages/GHC-59692/multipleInstances/before/MultipleInstances.hs

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+module MultipleInstances where
+
+instance Semigroup Int where
+    (<>) = (+)
+
+instance Semigroup Int where
+    (<>) = (*)

message-index/messages/GHC-59692/multipleInstances/index.md

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+---
+title: Multiple Instances for Semigroup Int
+---
+
+It is not possible to give two definitions for `Semigroup Int`, one for the semigroup given by addition and one for the semigroup given by multiplication.
+If we require both instances, then we should define two `newtype` wrappers for `Int`.