I'd start with a complete 'detail' strip of the 1911 pistol. Give every part a thorough cleaning. Inspect the hammer hooks & contact edge of the sear to make sure not worn or anything chipped off, etc.
If ok then, reassemble, including making sure the 3 leaf spring has the correct bends in it. Function test [empty] and ensure the hammer isn't falling when safety disengaged +/or hammer falling when slide released. If working properly, then I'd range test it with only 2 rounds in the mag. Load one & test fire for extract; eject; re-feed.
If hammer still falls after re-assembly then probably a new hammer & or sear is in order. As stated, on a 1911 this is not usually a 'drop in' fix.
Good luck.
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NAA.