Is there a way of acknowledging someones reply other than by adding to the original thread? I often post a thanks many hours after the person has posted but they are unlikely to be still monitoring the thread, and so don't receive their appreciation.
It's a good question, but on balance you waste precious space on the first index page by posting twice.
I can't speak for others, but I usually go through any questions to which I have replied in case there is any follow up query.
There has been a lot of debate on here about whether or not to say thankyou, but my personal opinion is that a small thankyou on the original post gives both thanks and an acknowledgement of closure for the question.