El gorilla siniestro

The full reasons behind the similarity of these two stories are given in German Wikipedia, but not in the English version, so I'll give a quick summary here for non-German speakers.

In mid-May 1968 a script for "Der Gorilla von Soho" had been written and the film was given the go-ahead, shooting to begin in June. The finished product was to be handed over in September.

However, for reasons unknown (perhaps problems with permission to film in Berlin Zoo or costs) that script was abandoned. With virtually no time to write a new one from scratch before filming was due to start, the producer and director decided to grab an old one - Dead Eyes of London - and essentially remake it, making changes to fit in with the Gorilla title.