Doubling in thirds or sixths
Doubling melodies in thirds or sixths (e.g. first violins playing the main melody while 2nd violins double in a third below) is in general something that is musically quite effective as additionally to providing melodic information it also gives a harmonic impression, however it is quite problematic on the taste level as it very often sounds cheesy and very old fashioned. This is especially problematic on melodies that move rhythmically quite quickly while it is practically no problem on melodies that are rather slow moving. It gets even more problematic if these two voices are texturally exposed. If you double in complete chords with more instruments, it is stylistically not that problematic however might become quite thick on the orchestration side of things.