In light of the answers from other people, I'd like to offer some basic math for how likely this is to occur.
The scenario I'm considering is as follows: You, an ordinary human, have two children. Those children have a child together (yes, I know incest is bad - this is purely hypothetical).
You, as an ordinary human, possess 46 chromosomes in 23 pairs. When you have a child, they receive one of each of these pairs from you (assuming nothing strange1 happens). As a side note, barring any medical breakthroughs that I'm not aware of, you would have to be genetically male (XY) so that your two offspring would be able to have children with each other.
For simplicity, I'll call the set of chromosomes that your first child got from you 'set A' and those that your second child got from you 'set B'. 882
In order for your grandchild to be an exact genetic duplicate of you, set B must contain all the chromosomes not found in set A, and then both children must pass on those specific chromosomes to their child. (This assumes that you do not have any chromosome pairs where both chromosomes are identical, which you probably don't unless you're the result of inbreeding).
Now that we have established the scenario, let's talk about the probabilities:
Let's assume the chance of someone's child receiving one specific chromosome from a pair is 1/2, as there are two possible outcomes.
With 23 pairs, the likelihood of passing on any one combination is 1/(2^23) (roughly one in 8 million). In our scenario, we see this happening 3 times: you passed on set B to your second child, your first child passed on set A to your grandchild, and your second child passed on set B to your grandchild. 2
So, the likelihood that your grandchild would be a genetic duplicate of you is 1/(2^69), or roughly one in 590 quintillion.
1 By "nothing strange", I mean there are no errors in the replication of the chromosomes, and nothing along the lines of having 3 chromosomes of the same type, such as the case of Triple X syndrome, or anything else I haven't thought to list here.
2 If you're wondering why I don't count when you passed set A on to your first child, it's because there are actually 2^23 different set A's that are possible, so the math cancels out. This is based off the assumption that there is already some set A you passed on to your first child. 882