These are two different words with the same letters which differ completely in meaning.

Duly is pronounced “dewly” while “dully” is pronounced with the “uh” sound.

To do something “dully” is to do it in a dull or boring manner. Too often people use this word when they mean “duly,” which means “properly.” Something duly done is done properly; something done dully is just a bore.

“Duly” is an adverb, meaning doing something in the proper or expected way, or at the correct or expected time, or in accordance with what is required or appropriate.

• The children were duly pleased with their presents.  (The children, as expected, were pleased with their presents.) 
• The deed was duly signed by the seller before we could occupy the premises.  (The deed was properly signed by the seller ….) 
• The school children duly assembled to hear the address by the principal.  (The school children, as was expected, assembled to hear the address by the principal.) 
• The entire cast of the play was overwhelmed by the applause and duly reappeared to take a third bow.  (The entire cast of the play was overwhelmed by the applause and as was appropriate, reappeared to take a third bow.) 
• The meeting came to an end with all objections duly noted in the minutes.  (The meeting came to an end with all objections, as was appropriate, being noted in the minutes.) 
• The taxi duly arrived at 9.30am to take us to the airport.  (The taxi arrived as expected at 9.30am to take us to the airport.) 
• The author duly signed all his books at the launch of his latest work.  (The author, as required, signed all his books at the launch of his latest work.) 
• The electricity bill was duly paid in time.  (The electricity bill was as required, paid in time.) 

“Dully” is an adverb derived from the adjective “dull”.  It signifies in a dull way, without liveliness, without lustre or shine. Below are some examples.

• The car lights glowed dully in the fog.  (Due to the fog, the car lights had no shine,) 
• Ever since I fell down, my back aches dully.  (Ever since I fell down, I have a nagging pain in my back.) 
• When the teacher asked him why he was late, Peter made no reply, but stood looking about him dully.  (When the teacher asked him why he was late, Peter made no reply but stood looking about him in a bored manner.) 
• She forced herself to look dully at the picture of her prospective husband.  (She forced herself to look uninterestedly at the picture of her prospective husband.)
• Life progressed dully at the ashram, since no entertainment was allowed.  (Life progressed in a boring manner at the ashram, since no entertainment was allowed.) 
• The gunshot sounded dully above the humming and roaring of the machines.  (The gunshot did not sound too loud above the humming and roaring of the machines.) 
• When asked why he chose to practice medicine, John dully responded: “Because my father wanted me to be a doctor.”  (When asked why he chose to practice medicine, John’s indifferent response was: “Because my father wanted me to be a doctor.”)
• I suppose I shall have to go to the funeral, she said dully.  (I suppose I shall have to go to the funeral, she said disinterestedly.)
• “My mother died when I was seven” he said dully.  (“My mother died when I was seven” he said without liveliness.)

Too often people confuse the word dully with duly.  Above examples illustrate they are quite different in meaning.

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