There are many words having similar meanings like talking and conversation, looking seeing watching .... etc!

There is no huge difference, the word "often" is used in conversation more than frequently. "I visite the museums in the city often", "Often times, new movies are sold out in theaters". Whereas "frequently" is usually used to describe the degree of how often something happens but isn't used in conversation more than often. These two words mean the same thing but "often" is more conversational than "frequently".