SAP智力题 Bird Shit II

When seven guys were walking, some birds flew over and left shits on their heads. Everyone can see shits on other people’s heads, but cannot see that on his own head. They entered a coffee shop, the seller said: “The count of shits on each of you is between 1 and 10, and there are two pairs among you, in each pair the counts of shits are same, (e.g. seven guys A,B,C,D,E,F,G, the count of shits on A is same as that on B, the count on C is same as that on F), and the others have different counts. You guys must not communicate. If anyone can make a correct guess of his own count, I will provide free coffee to all of you.” If the seven guys could imagine the situation and could have a discussion before hand, what strategy they may implement?