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).
Whether you are playing double stops or not it is very important to have a good hand position at all times to allow you to change finger positions quickly and precisely. If you have a bad hand position you may get away with it some of the time but you will have problems when you have to play a double stop or change quickly between adjacent strings.
To this end you need to keep your hand in a relaxed, cupped position with the fingers curved round and the last joints of the fingers as near vertical above the strings as possible. This can be difficult to achieve with the little finger over the G string but you need to get as close possible here. To this end you also need to keep the part of your hand below the little finger close to the neck of the violin at all times. If you don't do that then it is going to take a lot of movement to get the fourth finger down on the G string when you need it.
Once you have a good cupped hand position you then need to work on your finger contact point on the string. When you hold down a string with a finger it should be the left side of the tip of the finger (the part nearest your thumb) which contacts the string. This will give you the "cleanest" contact with the string and minimise the chance of accidentally contacting the strings on either side with resultant likelihood of a bad sound if you are either playing a double stop or have to play the next note on an adjacent string