Lapenna,
WELCOME TO THE AWS WELDING FORUM!!
The weld symbol only needs to be on both sides of the reference line if the design requires weld on both sides of the joint in question. It NEVER gets mirrored if it is not designated as a triangle (fillet weld symbol) on BOTH sides of the line. It may be a chain or a intermittent but if it is called out. If the Fillet Weld triangle is on both sides then it needs the length/pitch needs to be stated unless it is a continuous fillet AND according to A2.4 Clause 5.4.3 states the length/pitch numbers MUST be on both sides of the line.
Lawrence and others have already referenced you to A2.4. These are not decisions to be based upon personal opinion or guesswork. Go by the applicable codes.
You can also find brief descriptions of weld/welding symbols in many other AWS documents: A3.0 Terms and Defs, several of the code books, WIT, etc.
There, I gave you way too much information instead of making you look it up yourself. BUT, go to the bookstore and get the spec book.
He Is In Control, Have a Great Day, Brent