Three way switch with two common wires?
Notice how the two terminals are brass?  Those are travelers.  You'll save yourself a world of confusion if you get a 5-color pack of electrical tape and mark the traveler wires yellow.  Every 3-way switch circuit is different.  Colors are totally inconsistent in 3-way switches, the next one you see might have red and white travelers. 

It's not unusual to have 2 wires on the common terminal; that is simply the supply being split off to also supply something else.  That is more clear if it uses a pigtail, and feel free to convert it to a pigtail if you prefer. 

You have to be especially careful with smart-switch 3-ways.  Every 3-way switch configuration is different. There are many cases where the smart switch is not going to work in that box or even that application. Move slowly, document carefully, and if you get stuck on one, ask us a question and post pictures of what's going on inside both switch boxes.