I don't doubt it
no tengo ni una duda.
está bien o no?
If you type in dudar at this dictionary, you will find this:
No lo dudo. - I don't doubt it.
No cabe duda alguna - There is no doubt about it Is an expression that is related to what you are saying.
No tengo duda.
Because if you say No tengo ni una duda it would mean I don't have even a single doubt...
no tengo duda