Check for duplicate keys in config file

tests/bdd/features/config_file.feature

@@ -103,3 +103,8 @@ Feature: Multiple journals
         And we use the config "basic_onefile.yaml"
         When we run "jrnl --cf empty_file.yaml"
         Then the error output should contain "Unable to parse config file"
+
+    Scenario: Show a warning message when the config file contains double keys at the same level
+        Given we use the config "double_keys.yaml"
+        When we run "jrnl -1"
+        Then the output should contain "One or more keys appear multiple times at the same level"

tests/data/configs/double_keys.yaml

@@ -0,0 +1,27 @@
+default_hour: 9
+default_minute: 0
+editor: ''
+encrypt: false
+highlight: true
+template: false
+template: false
+journals:
+  default: 
+    encrypt: false
+    journal: features/journals/simple.journal
+    journal: features/journals/simple.journal
+  ideas:
+    encrypt: false
+    journal: features/journals/does-not-exist.journal
+  simple: 
+    encrypt: false
+    journal: features/journals/simple.journal
+    encrypt: false
+  work:
+    encrypt: false
+    journal: features/journals/work.journal
+linewrap: 80
+tagsymbols: '@'
+editor: nano
+timeformat: '%Y-%m-%d %H:%M'
+indent_character: "|"