What is the meaning of the phrase 'Once and again'?

Once and again: 
English Meaning: now and again.
Bangla Meaning: মাঝেমধ্যে। 

Example Sentence: Once and again this sort of thing will happen.
Bangla Meaning: এ ধরনের ঘটনা মাঝে মাঝেই ঘটবে।

• অন্য অপশন গুলোর মধ্যে - 
• Continually - On and on
English Meaning: continually.
Bangla Meaning: না থেমে; ক্রমাগত

• Very rarely - Once in a blue moon
English Meaning: very rarely.
Bangla Meaning: কদাচিৎ

• A long time ago - Once upon a time
English Meaning: used at the beginning of children's stories to mean "a long time ago".
Bangla Meaning: (বিশেষত শিশুকিশোর ও লোককাহিনি শুরুর প্রথাসিদ্ধ রীতি) এককালে

Source: Website lecture.