Is there a resource or sefer that lists halachic/legal principles found in the Gemara?
Examples of the principles I am referring to includes, מצות עשה דוחה לא תעשה, עוסק במצוה פטור במצוה, etc.
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It sounds to me that what you're looking for is the Encyclopedia Talmudis. It's a modern work which was begun something like 80 years ago by a group of people and is currently about halfway complete. (It goes alphabetically and they are in the middle/end of מ). It's extremely well organized and they cover really every detail of each concept, going from the shitos in the tanaim, amoraim, rishonim and acharonim. The only trouble is it's kind of expensive. Although, if you have a Bar Ilan usb, you can get the whole set added for much cheaper.