made duplicated fields and keys error for safety

src/parser.jl

@@ -87,9 +87,13 @@ field!(parser, dict) = begin
     while token == ","
         token = next_token!(parser, "a new entry or }")
         if token != "}"
-            key = token
+            key = lowercase(token)
+            if haskey(dict, key)
+                error("Duplicated field $key $(location(parser))")
+            else
                 expect!(parser, "=")
-            token, dict[lowercase(key)] = value!(parser)
+                token, dict[key] = value!(parser)
+            end
         end
     end
     expect(parser, token, "}")
@@ -139,6 +143,29 @@ julia> parse_bibtex("@book@")
 ERROR: Expected { on line 1
 [...]
 ```
+
+Repeated fields and keys are not allowed:
+
+```jldoctest
+julia> using BibTeX: parse_bibtex
+
+julia> parse_bibtex(""\"
+            @book{abook,
+                title = A}
+            @book{abook,
+                title = B}
+        ""\")
+ERROR: Duplicated id abook on line 3
+[...]
+
+julia> parse_bibtex(""\"
+            @book{abook,
+                title = A,
+                title = B}
+        ""\")
+ERROR: Duplicated field title on line 3
+[...]
+```
 """
 parse_bibtex(text) = begin
     parser = parse_text(text)
@@ -155,10 +182,15 @@ parse_bibtex(text) = begin
                     field!(parser, parser.substitutions)
                 else
                     id = next_token!(parser)
+                    records = parser.records
+                    if haskey(records, id)
+                        error("Duplicated id $id $(location(parser))")
+                    else
                         dict = Dict("__type__" => record_type)
                         expect!(parser, ",")
                         field!(parser, dict)
-                    parser.records[id] = dict
+                        records[id] = dict
+                    end
                 end
             end
         end