When you import movies into iMovie 10 libraries, the file is always copied, wasting space and hindering editability. This script replaces the copy with a symlink, reclaiming disk space.

dedup-iMovie-library.py

#!/usr/bin/env python3
# 
# Usage: dedup-imovie-library.py LIBRARY ORIGINALS
#
# Public gist:
# https://gist.github.com/miguelmorin/92cd8b820fd0135bf0f7d5fea90dc3f1
#
# This code was inspired by this post and gist, and converted to Python
# https://benhollis.net/blog/2014/06/28/hard-linking-videos-to-save-space-in-your-imovie-library/
# https://gist.github.com/bhollis/6682c8b6b6357e2fedc5
# 
# Goes through an iMovie 10 library and replaces all the "Original Media" movie
# and image files with symlinks to the actual original media, in order to conserve
# disk space.
# 
# This assumes you've already imported the files into iMovie and waited for them
# all to be copied.
#
# You can adapt the global variable `PROJECTS_TO_SKIP` to avoid replacing media
# on some projects that you may be working on.
# 
# This assumes that your iMovie library and originals folder are organized by
# the same event name (because sometimes a camera records similarly named
# DSC001.MOV and one needs a way to distinguish them). If the names are
# different, e.g. if you create two events titled "movie" then iMovie renames
# the second to "movie 1", you can adapt the global variable
# `SHOW_NAME_CORRESPONDENCE` to map the name of the iMovie event to the name of
# the folder with the original content.
# 
# author: Miguel Morin
# copyright: public domain
# year: 2019

import doctest
import glob
import os
import pathlib
import shutil
import sys

FILE_SUFFIXES_LOWERCASE = [".mp4", ".mts", ".mov", ".jpg", ".jpeg", ".png"]

PROJECTS_TO_SKIP = []  # e.g., ["project 1", "project 2"]

SHOW_NAME_CORRESPONDENCE = {}  # e.g. {"movie": "movie 1"}

def skip(f):
    """Returns a boolean for whether to skip a file depending on suffix.
    >>> skip("abc.mp4")
    False
    >>> skip("ABC.JPEG")
    False
    >>> skip("abc.plist")
    True
    >>> skip("00114.MTS")
    False
    """
    suffix = pathlib.Path(f).suffix.lower()
    return suffix not in FILE_SUFFIXES_LOWERCASE

def get_show_and_name(f):
    """
    >>> show, name = get_show_and_name("/Volumes/video/iMovie Library.imovielibrary/my great show/Original Media/00117.mts")
    >>> "my great show" == show
    True
    >>> "00117.mts" == name
    True
    >>> show, name = get_show_and_name("/Volumes/video/path/to/originals/my great show/00117.mts")
    >>> "my great show" == show
    True
    >>> "00117.mts" == name
    True
    """
    path = pathlib.Path(f)
    name = path.name.lower()
    
    dirname = str(path.parents[0])

    imovie = "iMovie Library.imovielibrary" in dirname
    parent_dir = str(path.parents[2 if imovie else 1])
    show = dirname.replace(parent_dir, "")

    if imovie:
        assert show.endswith("/Original Media"), f
        show = show.replace("/Original Media", "")

    assert show.startswith("/")
    show = show[1:].lower()
    
    if show in SHOW_NAME_CORRESPONDENCE:
        show = SHOW_NAME_CORRESPONDENCE[show]

    return show, name

def build_originals_dict(originals):
    """Go through the original directory to build a dictionary of filenames to paths."""
    originals_dic = dict()

    for f in glob.glob(os.path.join(originals, "**", "*.*"), recursive=True):
        if skip(f):
            continue

        show, name = get_show_and_name(f)
        
        originals_dic[(show, name)] = f

    return originals_dic

def replace_files_with_symlinks(library, originals):
    """Go through the iMovie library and find the replacements."""
    originals_dic = build_originals_dict(originals)

    # List files recursively
    for f in glob.glob(os.path.join(library, "**", "*.*"), recursive=True):
        if skip(f) or os.path.islink(f):
            continue

        show, name = get_show_and_name(f)

        if (show, name) in originals_dic:
            target = originals_dic[(show, name)]

            print("Replacing %s with %s" % (f, target))
            os.unlink(f)
            os.symlink(target, f)
        else:
            print("No original found for %s" % f)

def main():
    args = sys.argv
    assert 3 == len(args), "You need to pass 3 arguments"
    library = args[1]
    originals = args[2]

    replace_files_with_symlinks(library = library, originals = originals)
        
if "__main__" == __name__:
    r = doctest.testmod()
    assert 0 == r.failed, "Problem: doc-tests do not pass!"

    main()        

@bmaupin: Yes, with two other advantages: it removes the duplicated file and creates a symlink in its place so the iMovie library is completely functional and all the projects still work.