That part of a sentence is correct and usable in written English. You can use it when you are expressing appreciation for something someone has done for you. For example: "Once again, thank you for helping me with the project - I couldn't have done it without you.".
Once again, thank you for your letter.
We once again thank you for your patience".
Lesley Model, London College of Communication Once again thank you for having us.
At the final taping of her talk show a couple days ago, she teared up and said: I've never experienced anything like this and I say once again thank you for taking me to a place that's beyond joyous.
So once again thank you, David Bowie, for the joy of courage against the stranglehold of fear that death can have over us.
Once again, thank you so much for supporting me".
Once again, thank you so much for a really wonderful weekend.
I love the desktop app, it’s always running on my Mac. Ludwig is the best English buddy, it answers my 100 queries per day and stays cool.
Cristina Valenza
Retail Lead Linguist @ Apple Inc.