I am new to double stops. When I play a double stop where a finger is on a string while playing an open string (1st finger on the D string and open A string), my finger touches the open string, in turn getting a scratchy sound from it. Is there a simple method to not let my finger from touching the open string?
Work with a teacher. (Yeah, I know, I always answer with that suggestion). Every person learning a bowed string instrument goes thru this at some point: it's a matter of learning finger shape and position, and that's best done with a teacher or other experienced player watching you.
In fact, you'll have to learn not to touch neighboring strings most of the time, so as not to interfere with a fingered position on the next string. For example, placing your 3rd finger on the lower of two strings and your second on the higher -- if 3rd finger touches the upper string, you won't get the desired fingered note on that string (and proper technique in many passages requires holding 2 or more fingers in place while bowing from one string to another and probably back again).