Add provide-deduplication rule

Closes #28. The implementation is pretty basic and ignores things like `(provide (for-syntax x x))` but it catches the simple case.

default-recommendations.rkt

@@ -22,6 +22,7 @@
                resyntax/default-recommendations/match-shortcuts
                resyntax/default-recommendations/miscellaneous-suggestions
                resyntax/default-recommendations/numeric-shortcuts
+               resyntax/default-recommendations/require-and-provide-suggestions
                resyntax/default-recommendations/string-shortcuts
                resyntax/default-recommendations/syntax-shortcuts
                resyntax/default-recommendations/syntax-parse-shortcuts
@@ -48,6 +49,7 @@
          resyntax/default-recommendations/match-shortcuts
          resyntax/default-recommendations/miscellaneous-suggestions
          resyntax/default-recommendations/numeric-shortcuts
+         resyntax/default-recommendations/require-and-provide-suggestions
          resyntax/default-recommendations/string-shortcuts
          resyntax/default-recommendations/syntax-shortcuts
          resyntax/default-recommendations/syntax-parse-shortcuts
@@ -86,6 +88,7 @@
            (refactoring-suite-rules match-shortcuts)
            (refactoring-suite-rules miscellaneous-suggestions)
            (refactoring-suite-rules numeric-shortcuts)
+           (refactoring-suite-rules require-and-provide-suggestions)
            (refactoring-suite-rules string-shortcuts)
            (refactoring-suite-rules syntax-shortcuts)
            (refactoring-suite-rules syntax-parse-shortcuts)

default-recommendations/require-and-provide-suggestions-test.rkt

@@ -0,0 +1,56 @@
+#lang resyntax/testing/refactoring-test
+
+
+require: resyntax/default-recommendations require-and-provide-suggestions
+
+
+header:
+- #lang racket/base
+
+
+test: "duplicate provided identifiers should be removed"
+----------------------------------------
+(provide foo
+         foo
+         foo)
+(define foo 1)
+----------------------------------------
+----------------------------------------
+(provide foo)
+(define foo 1)
+----------------------------------------
+
+
+test: "removing duplicate provided identifiers leaves other exports unchanged"
+----------------------------------------
+(provide a
+         foo
+         b
+         foo
+         c)
+(define a 1)
+(define b 1)
+(define c 1)
+(define foo 1)
+----------------------------------------
+----------------------------------------
+(provide a
+         foo
+         b
+         c)
+(define a 1)
+(define b 1)
+(define c 1)
+(define foo 1)
+----------------------------------------
+
+
+test: "provide deduplication doesn't affect exports at different phases"
+----------------------------------------
+(provide foo
+         (for-syntax foo))
+(require (for-syntax racket/base))
+(define foo 1)
+(begin-for-syntax
+  (define foo 2))
+----------------------------------------

default-recommendations/require-and-provide-suggestions.rkt

@@ -0,0 +1,43 @@
+#lang racket/base
+
+
+(require racket/contract/base)
+
+
+(provide
+ (contract-out
+  [require-and-provide-suggestions refactoring-suite?]))
+
+
+(require racket/list
+         rebellion/private/static-name
+         resyntax/refactoring-rule
+         resyntax/refactoring-suite
+         syntax/parse)
+
+
+;@----------------------------------------------------------------------------------------------------
+
+
+(define-syntax-class export-spec
+  #:attributes (id)
+  (pattern id:id))
+
+
+(define-refactoring-rule provide-deduplication
+  #:description "Providing the same identifier multiple times is unnecessary."
+  #:literals (provide)
+  (provide spec:export-spec ...)
+  #:when (check-duplicate-identifier (attribute spec.id))
+
+  #:with (deduped-spec ...)
+  (remove-duplicates (attribute spec) bound-identifier=?
+                     #:key (λ (spec-stx) (syntax-parse spec-stx [:export-spec #'id])))
+
+  (provide deduped-spec ...))
+
+
+(define require-and-provide-suggestions
+  (refactoring-suite
+   #:name (name require-and-provide-suggestions)
+   #:rules (list provide-deduplication)))