Accept duplicate entries in INI config files

Rationale:
1. The `-D` command line option can be specified multiple times.
   It makes sense to be able to specify the respective config file
   option `dictionary` multiple times too.
2. While spcifying an option multiple times might be an error, the
   setting strict to `True` catches duplicates only within a single
   INI file, not across multiple INI files. If we are serious about
   handling duplicate options, we should therefore handle that
   manually, distinguishing options that can be duplicated or not.

codespell_lib/_codespell.py

@@ -541,7 +541,7 @@ def parse_options(
     cfg_files = ["setup.cfg", ".codespellrc"]
     if options.config:
         cfg_files.append(options.config)
-    config = configparser.ConfigParser(interpolation=None)
+    config = configparser.ConfigParser(interpolation=None, strict=False)
 
     # Read toml before other config files.
     toml_files = []
@@ -571,7 +571,7 @@ def parse_options(
     # Collect which config files are going to be used
     used_cfg_files = []
     for cfg_file in cfg_files:
-        _cfg = configparser.ConfigParser()
+        _cfg = configparser.ConfigParser(interpolation=None, strict=False)
         _cfg.read(cfg_file)
         if _cfg.has_section("codespell"):
             used_cfg_files.append(cfg_file)