Add documentation for GHC-05661

Author: David Binder

message-index/messages/GHC-05661/doubleQualified/after/DoubleQualified.hs

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+{-# LANGUAGE ImportQualifiedPost #-}
+module DoubleQualified where
+
+import Prelude qualified

message-index/messages/GHC-05661/doubleQualified/before/DoubleQualified.hs

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+{-# LANGUAGE ImportQualifiedPost #-}
+module DoubleQualified where
+
+import qualified Prelude qualified

message-index/messages/GHC-05661/doubleQualified/index.md

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+---
+title: The module Prelude is incorrectly imported as qualified
+---
+
+In this example, the `qualified` modifier was used both before and after the name of the module which is imported.
+This is not legal syntax, and GHC emits this error message.
+The error can be fixed by removing one of the two occurrences.
+
+```
+messages/GHC-05661/doubleQualified/before/DoubleQualified.hs:4:26: error: [GHC-05661]
+    Multiple occurrences of 'qualified'
+  |
+4 | import qualified Prelude qualified
+  |                          ^^^^^^^^^
+```

message-index/messages/GHC-05661/index.md

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+---
+title: Cannot use qualified in pre and postpositive position
+summary: A module cannot be imported using the qualified modifier in pre and postpositive position
+severity: error
+introduced: 9.6.1
+---
+
+With the language extension `ImportQualifiedPost` it is possible to import a module using the `qualified` modifier after the name of the module. But it is not legal to use the `qualified` modifier both before and after the name of the module.