'Doubt' is generally a stative verb - like 'believe', for example, - which tends to be used in the present simple. This is because it describes a state rather than an activity. So it would be more usual to say 'I doubt whether...'. It's possible to say 'I'm doubting whether...', but only if you want to emphasise the idea that you are just doubting at this precise moment, or only just beginning to doubt.

You're talking about the present simple and the present continuous, not the perfect tenses. "I doubt" is more certain than "I'm doubting whether". I doubt suggest that you don't think you will get another chance, whereas I'm doubting means that you're not sure.

Hi, we would use "Doubt". For example: I doubt that I'll get a 100 on the exam. "I doubt whether or not I will get another chance to retake the exam."

Hi! what's up? "Doubt" is a state verb like belive; consist of; doubt; own. This verbs are rarely used in present progressive (also called by present continuous). Therefore, the correct option is to use the present simple. Infinitive (I, we,you,they) or infinitive (he, she, it) plus "s".