In most cases we've seen, if the images are loading twice, it's because of a JavaScript event that's firing multiple times.

It can also happen if you select a thumbnail in the WebThumbnailViewer after you've already opened the same image in the WebImageViewer/WebAnnotationViewer.  If the WebThumbnailViewer you're using is bound to the WebImageViewer, and any images are selected, it will auto-load the selected image on page load... there is no need to open the first image in the WebImageViewer, as the WebThumbnailViewer is controlling what it loads.