Fix issue with repeated files on Windows

As we now find files using a glob for each individual file extension,
case insensitive filesystems will find the same file for both
extensions that only differ by case.

An alternative fix for this might be to squash all the extensions into
a single glob pattern and run that in each directory

ford/fortran_project.py

@@ -25,7 +25,7 @@
 import os
 import toposort
 from itertools import chain, product
-from typing import List, Optional, Union, Dict
+from typing import List, Optional, Union, Dict, Set
 from pathlib import Path
 from fnmatch import fnmatch
 
@@ -84,7 +84,7 @@
 }
 
 
-def find_all_files(settings: ProjectSettings) -> List[Path]:
+def find_all_files(settings: ProjectSettings) -> Set[Path]:
     """Returns a list of all selected files below a set of directories"""
 
     file_extensions = chain(
@@ -94,16 +94,16 @@ def find_all_files(settings: ProjectSettings) -> List[Path]:
     )
 
     # Get initial list of all files in all source directories
-    src_files: List[Path] = []
+    src_files: Set[Path] = set()
 
     for src_dir, extension in product(settings.src_dir, file_extensions):
-        src_files.extend(Path(src_dir).glob(f"**/*.{extension}"))
+        src_files.update(Path(src_dir).glob(f"**/*.{extension}"))
 
     # Remove files under excluded directories
     for exclude_dir in settings.exclude_dir:
-        src_files = [
+        src_files = {
             src for src in src_files if not fnmatch(str(src), f"{exclude_dir}/*")
-        ]
+        }
 
     bottom_level_dirs = [src_dir.name for src_dir in settings.src_dir]
     # First, let's check if the files are relative paths or not
@@ -122,9 +122,9 @@ def find_all_files(settings: ProjectSettings) -> List[Path]:
             settings.exclude[i] = glob_exclude
 
     for exclude in settings.exclude:
-        src_files = [
+        src_files = {
             src for src in src_files if not fnmatch(os.path.relpath(src), exclude)
-        ]
+        }
 
     return src_files