Dueling Dubs: On the short-list of anime films with the most English dubs made (often attributed as #1† though that's technically incorrect – the actual record belongs to the first episode of Dragon Ball, which has six different known dubs, most of which are lost… so Mamo may still hold the record for most extant dubs). First, William Ross, whose Frontier Enterprises company was hired for the job by Japanese distributor Toho, made a dub in 1979 to show on Japan Airlines trans-Pacific flights. Next, Streamline Pictures dubbed it in 1995 for the North American market. Then, Manga UK made its own dub in 1996 for the PAL market for reasons discussed below. Finally, Pioneer/Geneon redubbed the film in 2003 using its cast from "Red Jacket".
In 2013, rather than trying to make a dub of their own, Discotek Media simply and wisely collected all four dubs onto their release. This was actually quite a feat, since the Toho dub had to be cobbled together from various extant incomplete copies and then digitally cleaned up to make it presentable, and the Manga UK dub had a very limited release to the point that no one knew if decent materials for it even existed anymore (decent enough, though it also had to be digitally cleaned up).
Fake American: Nearly everyone in Manga UK's version, one exception being Lupin's voice actor, William Dufris (who actually is American).
Franchise-Driven Retitling: In Japan, The Mystery of Mamo was originally titled simply Lupin III, but with two television series, a live-action film, and another movie on the way, they had to retitle it to distinguish what the movie was. It is now officially known as Lupin III: Lupin vs. the Clones.
What Could Have Been: For their release, Streamline Pictures originally planned on simply including the pre-existing Toho dub, similar to how they released AKIRA. However, they weren't able to due to copyright issues with TMS or Toho, and instead had to produce their own dub.
Similarly, Manga UK could not use Streamline's dub due to its retention of Lupin's name and concerns about a lawsuit from across the Channel, so they had to make their own dub. That cast would return for their UK-only dub of the first special, Bye-bye Liberty Crisis.