This is in Vegas Pro 15, Build 321, under Windows 10, on the MSI GT72S 6QE Dominator Pro G. When listing the available FX, many of them are listed many times over. Please see the enclosed video and watch the upper left corner, where I am scrolling through the list. Right at the beginning, for example the Add Noise FX is listed more times than fit in the list window! Then there are BCC plugins, each listed only once, but then the Black and White and later the Brightness and Contrast FX are listed many times, etc.
It only seems to affect the FX that come with Vegas natively.
A similar problem (not shown in the video) happens when listing Media Generators.
The problem is really frustrating and highly annoying.
Adam