Is the following sentence correct?
I'll let you know tomorrow morning, if I will be coming tomorrow.
Yes, this is correct.
The structure that I think probus is talking about, and which we don't use in English is: "If I will come tomorrow, I will let you know." We don't normally use "will" in the 'if' clause. For example, "If you [STRIKE]will [/STRIKE] go to Japan, you will enjoy yourself". You can normally omit 'will' and the sentence still makes sense; that's one sign of a wrong "will".
However, that doesn't mean that "will" cannot appear in an 'if' clause.
These are correct sentences:
"If I'll be at the party tomorrow (If I decide to go to the part tomorrow), I'll let you know beforehand."
"If you will do this job, please let me know by tomorrow".