Raise exception when a duplicate key is found when reading from a file.

Fixes #34 (#36)

spec/parser_spec.cr

@@ -64,6 +64,12 @@ describe LuckyEnv::Parser do
       data["DB_NAME"].should eq "my_app_development"
       data["LITERAL"].should eq "${NOT_EXISTS_ENV}"
     end
+
+    it "raises on duplicate keys detected" do
+      expect_raises(LuckyEnv::DuplicateKeyDetectedError, /Duplicate key HOST found in \.\/spec\/support\/\.badenv/) do
+        parser.read_file("./spec/support/.badenv")
+      end
+    end
   end
 end
 

spec/support/.badenv

@@ -0,0 +1,2 @@
+HOST=localhost
+HOST=duplicate_key

src/lucky_env/errors.cr

@@ -4,4 +4,7 @@ module LuckyEnv
 
   class MissingFileError < Exception
   end
+
+  class DuplicateKeyDetectedError < Exception
+  end
 end

src/lucky_env/parser.cr

@@ -30,7 +30,16 @@ module LuckyEnv
           next if comment?(string)
           key, value = parse_value(string)
 
-          hash[key] = value
+          if hash.has_key?(key)
+            raise LuckyEnv::DuplicateKeyDetectedError.new <<-ERROR
+            Duplicate key #{key} found in #{file_path}.
+            To ignore a key, place a # at the front of the line like this:
+
+            # YOUR_KEY=ignored_value
+            ERROR
+          else
+            hash[key] = value
+          end
         end
 
         keys = hash.keys