Just: 刚 (gāng) and 刚刚 (gānggāng)

If you want to emphasize something "just" happened, you can use  刚 (gāng) or 刚刚 (gānggāng) to indicate it happened in a flash, or they happened a very short time before you talk about them. They can be used interchangeably, both 刚 (gāng) and 刚刚 (gānggāng) can express that the action happened not long ago. 

• Subject + 刚 + Verb 
• 他刚走。He just left.
• 我刚到。I just arrived. 
• Subject + 刚刚 + Verb
• 他刚刚走。He just left.
• 我刚刚到。I just arrived. 

However,  刚 (gāng)  can be used to describe something "Just" happened, with a specific time period: Subject + 刚 + VO + Duration. In this patter, 刚 (gāng) can express "from the time an action happens until the present time".

• 我刚到这里五分钟。I just got here five minutes ago.
• 他的宝宝刚出生两个星期。His baby was just born for two weeks.
• 我刚学中文一个学期。I've just learned Chinese for a semester. 
• 小张刚来美国一个月。Little Zhang just arrived in America a month ago. 

Please fill in the blanks.

1. 我_____来这个手机两个礼拜，可是它就坏了。I just got this phone for two weeks, but now it's broken.
2. 我_____听到这个消息。I just hear this news. 
   
3. 他_____来中国一天，还不太会用筷子。He just came to China for one day. He does not know how to use the chopsticks.
4. 她_____来我们公司两周。She just came to our company for two weeks. 
5. 他们_____去学校。They just went to school. 

Reference:
http://resources.allsetlearning.com/chinese/grammar/%22Just%22_with_%22gang%22