In Blender:
1. Select the object;
2. In the edit buttons panel, click on
Add Modifier->
Mirror, choose how the mirror will be created and
Apply it;
3. In edit mode, press
A to select all of the object's vertices;
4. Press
W to open the "
Specials" menu. Then click on "Remove doubles" (or just press 6 if you are used to short cuts).
Instead of doing the steps 3 and 4, you may just increase the "Merge limit" property of the mirror modifier at step 2 before applying it. Blender shows the result of the modifier in real time, but you need to practice this before using on dense and detailed meshes.
"The difference between stupidity and genius is that genius has its limits." - Albert Einstein
