Having two items with an identical icon, identical name, but coming from different places can be incredibly confusing (not to mention, as someone who has to work on the GE Wiki and keep things up to date, almost impossible to deal with for items :P) so is there any chance that this item can have its name changed slightly somehow? Maybe even something simple like... Deep-sea Spider Crab.
The first Spider Crab you can get from Spearfishing, and it was added to the game in Patch 4.0 (Item ID# 20154)... Here's the Gamer Escape entry on it: https://ffxiv.gamerescape.com/wiki/Spider_Crab (at the moment, this page lists both methods of getting this item due to how the GE wiki does things, by name, so both the spearfishing and the deep sea voyage thing shows up on it.)
The other Spider Crab was added this patch... and is supposed to come from the new Deep Sea fishing/ocean voyage event. (Item ID# 29741)
This is the very first item in the game to be in this situation, with both being acquired from Fishing somehow, both with the same name and same icon... I'm assuming this one just slipped through the cracks somehow, because it would be really good to not have this happen more often in the future D:
For what it's worth, the other languages all have these items named differently from each other. It's only English that has this problem. Here's a rather crappily formatted list of the names in the different languages for each Item ID#
Code:Lang. Item #20154 Item #29741 DE Riesenkrabbe Spinnenkrabbe FR Crabe araignée Crabe-araignée géant JP タカアシガニ スパイダークラブ EN Spider Crab Spider Crab
EDIT: This was fixed in Patch 5.21. The new crab item (29741) is called "Net Crawler". Thank you for fixing this!