SIM808 return the same AT command
My default modems usually echo back whatever you send.  So if you send AT it will first respond with AT followed by the OK response:

&gt;&gt; AT
&lt;&lt; AT
&lt;&lt; OK


Echoing can be turned off with the command ATE0 and turned back on with ATE1:

&gt;&gt; ATE0
&lt;&lt; ATE0
&lt;&lt; OK
&gt;&gt; AT
&lt;&lt; OK
&gt;&gt; ATE1
&lt;&lt; OK
&gt;&gt; AT
&lt;&lt; AT
&lt;&lt; OK