I think naming someone as Kirk is probably forgivable (although you could always say, "When I went out with a Star Fleet Captain..."), but generally a punning joke is less satisfying if one of the words has already been used. So, in the example I used above:
I used to love farm machinery, but now I'm an ex-tractor fan
is a lot more satisfying than
I used to be fan of tractors, but now I'm an - oh, you've worked it out, OK, bye then. 
Quote: gappy @ 14th December 2014, 11:38 AM GMTI think naming someone as Kirk is probably forgivable (although you could always say, "When I went out with a Star Fleet Captain..."), but generally a punning joke is less satisfying if one of the words has already been used.
that makes me sound soooooo old. I was thinking in terms of Kirk Douglas from the Vikings but he is about 93 now 
But it doesn't matter which Kirk
Another Tim Vine joke in the paper the other day.
My friend always wanted to be killed by a steam train. He was chuffed to bits.
I used to tell one about a terminally ill dog with no hind-legs, but it died on its arse
Quote: bushbaby @ 14th December 2014, 8:43 AM GMTYes, we used both terms up't north
Ahh, fair enough.
Quote: FunyHaHA Not Funy Strange @ 14th December 2014, 10:50 AM GMTI don't understand what you mean Tony sorry? can you give a few examples and why its a sin? I have the feeling I'm going to be one of the guilty ones, but don't know what I've done! lol :-)
I think gappy has covered this but the reason why it's a sin is that it signposts the punchline and is less satisfying.
For instance I wrote a joke which goes;
I bought some cheese which had the words "Wee" and "Poo" written through it, it was immature cheddar.
Now if I'd written the joke as;
I bought some cheddar cheese which had the words "Wee" and "Poo" written through it, it was immature cheddar.
It's less effective because I repeated the vital word from the punchline in the set up, in fact, if I'm being picky my example above repeats the word "it" which is a bit clunky, unless the joke relies on it I would always try to avoid repetition.
Synonyms are your friend, say your joke relies on the word "bike" in the punchline, you'd use "bicycle" or "cycle" in the set up. Sometimes you'll have to pull a bit of linguistic trickery but this is usually better than repeating a word.
I tried to use a lift the other day. It refused to let me in for being "too fat". I didn't notice the sign: machine is out of order.
Quote: Tony Cowards @ 14th December 2014, 11:41 PM GMTFor instance I wrote a joke which goes;
I bought some cheese which had the words "Wee" and "Poo" written through it, it was immature cheddar.
Now if I'd written the joke as;
I bought some cheddar cheese which had the words "Wee" and "Poo" written through it, it was immature cheddar.
It's less effective because I repeated the vital word from the punchline in the set up, in fact, if I'm being picky my example above repeats the word "it" which is a bit clunky, unless the joke relies on it I would always try to avoid repetition.
Synonyms are your friend, say your joke relies on the word "bike" in the punchline, you'd use "bicycle" or "cycle" in the set up. Sometimes you'll have to pull a bit of linguistic trickery but this is usually better than repeating a word.
A thesaurus is one's chum, amigo, buddy.
That cheddar cheese on cracked me up, alternatively there's one about mature cheddar and incontinence and/or false teeth there somewhere...
When I found out it was my best friend who stole my Thesaurus, I was lost for words!
Whay did the Jamaican police man shoot the cheese full of holes?
Because it's ementhal!
I think my Ethernet port and my router have fallen out.
They haven't talked to each other in weeks.
Quote: Stephen Goodlad @ 15th December 2014, 8:04 PM GMTI think my Ethernet port and my router have fallen out.
They haven't talked to each other in weeks.
No idea if that's clever or even funny!
Evidently neither
"Knock Knock."
"Who's there?"
"Objective Pronoun."
"Objective Pronoun who?"
"Objective Pronoun whom."
Quote: Stephen Goodlad @ 15th December 2014, 8:37 PM GMTEvidently neither
I laughed! (I would have responded sooner but had problems with my internet connection! I hate Virgins!)
Christmas shopping tip.
Never buy white underpants for a man over 40 years old.
Especially if you do his laundry.
P*ss off!!??? How uncouth! I'm a f*ckin' lady!