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1）美国居民从海外汇钱给另一个美国居民，是否有不能超过1万5千美金的免税额度？

The rule is: any gift (defined as from A to B) has no tax consequences if under $15,000.

Above 15,000, it doesn't mean you have to pay gift tax. 

Another rule is:  Life time gift tax exemption = estate tax exemption

Federal tax law allows a person leave $11.7 million to others without estate tax. This is called estate tax exemption.  You can use the exemption amount before you die or after your death.  If you give someone more than $15,000 a year, You can deduct the above 15,000 amount from your estate tax exemption amount, and still don't have to pay gift tax. 

He has to file form 709 for any gift above $15,000. 

2）这个额度是捐赠人的年度总额度还是给每个受赠人的额度？比如我父亲是美国绿卡，他今年从海外汇钱赠予我1.2万美金，那么他今年是否还可以再汇款1万美金给他的一个朋友？

Any gift is deinfed as a donation from A to B.  So he can give out $15,000 as much as he wants to different people.  However I do not recommend him to break up his gifts in order to evade gift tax.