Make strict warn on repeated citekeys

Signed-off-by: Marcello Seri <marcello.seri@gmail.com>

Author: mseri

CHANGES.md

@@ -1,3 +1,7 @@
+
+- bibfmt: in strict mode, print a warning if there are multiple
+  repeated citekeys (case-insensitive).
+
 # 0.7.7 (2025-08-07)
 
 - bibfmt and doi2bib drop the month field in bibtex. It is

bibfmt/lib/bibtex.ml

@@ -677,6 +677,31 @@ let format_bibtex_item options = function
 
 let pretty_print_bibtex ?options items =
   let options = Option.value options ~default:default_options in
+  (if options.strict then
+     (* Report all duplicate citekeys *)
+     let citekeys =
+       List.fold_left
+         (fun acc content ->
+           match content with
+           | Entry entry -> entry.citekey :: acc
+           | Comment _ -> acc)
+         [] items
+     in
+     (* Find duplicate citekeys with case-insensitive comparison *)
+     let lowercase_citekeys = List.map String.lowercase_ascii citekeys in
+     let rec find_duplicates seen acc = function
+       | [] -> acc
+       | key :: rest ->
+           if List.mem key seen then find_duplicates seen (key :: acc) rest
+           else find_duplicates (key :: seen) acc rest
+     in
+     let duplicates = find_duplicates [] [] lowercase_citekeys in
+     if duplicates <> [] then (
+       let duplicate_msg = String.concat ", " (List.rev duplicates) in
+       Printf.eprintf
+         "Warning: Duplicate citekeys found (case-insensitive): %s\n"
+         duplicate_msg;
+       flush stderr));
   String.concat "\n" (List.map (format_bibtex_item options) items)
 
 (* Helper function to clean and normalize BibTeX entries *)

bibfmt/tests/bibfmt.t/run.t

@@ -424,3 +424,109 @@ Test mixed UTF-8 with BibTeX special characters and escapes
     NOTE      = "Special: \&, \%, $, plus UTF-8: café, résumé, naïve, piñata",
     PUBLISHER = "Éditions Académiques & Co."
   }
+
+Test duplicate citekey detection with case-insensitive comparison in strict mode
+  $ cat > duplicates.bib << EOF
+  > @article{test1,
+  >   title = "First Article",
+  >   author = "John Doe",
+  >   year = 2020
+  > }
+  > 
+  > @book{Test1,
+  >   title = "Same Citekey Different Case",
+  >   author = "Jane Smith",
+  >   year = 2021
+  > }
+  > 
+  > @inproceedings{test2,
+  >   title = "Different Citekey",
+  >   author = "Bob Wilson",
+  >   year = 2022
+  > }
+  > EOF
+
+  $ bibfmt --strict -f duplicates.bib
+  Warning: Duplicate citekeys found (case-insensitive): test1
+  @article{test1,
+    TITLE  = "First Article",
+    AUTHOR = "John Doe",
+    YEAR   = 2020
+  }
+  
+  @book{Test1,
+    TITLE  = "Same Citekey Different Case",
+    AUTHOR = "Jane Smith",
+    YEAR   = 2021
+  }
+  
+  @inproceedings{test2,
+    TITLE  = "Different Citekey",
+    AUTHOR = "Bob Wilson",
+    YEAR   = 2022
+  }
+
+Test that non-strict mode doesn't warn about duplicate citekeys
+  $ bibfmt -f duplicates.bib
+  @article{test1,
+    TITLE  = "First Article",
+    AUTHOR = "John Doe",
+    YEAR   = 2020
+  }
+  
+  @book{Test1,
+    TITLE  = "Same Citekey Different Case",
+    AUTHOR = "Jane Smith",
+    YEAR   = 2021
+  }
+  
+  @inproceedings{test2,
+    TITLE  = "Different Citekey",
+    AUTHOR = "Bob Wilson",
+    YEAR   = 2022
+  }
+
+Test multiple duplicate citekeys in strict mode
+  $ cat > multiple_duplicates.bib << EOF
+  > @article{paper1,
+  >   title = "First Paper",
+  >   author = "Author One"
+  > }
+  > 
+  > @book{PAPER1,
+  >   title = "Book Version",
+  >   author = "Author Two"
+  > }
+  > 
+  > @inproceedings{conference1,
+  >   title = "Conference Paper",
+  >   author = "Author Three"
+  > }
+  > 
+  > @misc{Conference1,
+  >   title = "Same Conference Different Type",
+  >   author = "Author Four"
+  > }
+  > EOF
+
+  $ bibfmt --strict -f multiple_duplicates.bib
+  Warning: Duplicate citekeys found (case-insensitive): conference1, paper1
+  @article{paper1,
+    TITLE  = "First Paper",
+    AUTHOR = "Author One"
+  }
+  
+  @book{PAPER1,
+    TITLE  = "Book Version",
+    AUTHOR = "Author Two"
+  }
+  
+  @inproceedings{conference1,
+    TITLE  = "Conference Paper",
+    AUTHOR = "Author Three"
+  }
+  
+  @misc{Conference1,
+    TITLE  = "Same Conference Different Type",
+    AUTHOR = "Author Four"
+  }