Hello,
When a merge of LiDAR tiles either with the lasmerge tool or with the on-the-fly option '-merged' that is available in all LAStools emits a WARNING that the tiles you are merging are buffered then one should pay attention. Merging buffered tiles will in general be a mistake as the points in the buffer area will be appearing two or even four times in the merged tile with only one copy being "good" and three copies being "dirty" or "tile edge artifacted".
There are two ways of getting rid of the buffer points.
(1) Run lastile once more with the '-remove_buffer' option to generate a new set of tiles where the buffer points were removed.
(2) Use the option '-drop_withheld' but that requires three conditions: (a) you used lastile to generate the buffers, (b) you added the '-flag_as_withheld' option during the lastile process that created the buffered tiles, and (c) the 'withheld' flag is not used for other purposes in your LiDAR processing pipelines.
Also see this blog entry about buffers:
Regards,
Martin