I have set up an 1.6.4 server instance and noticed that an ID conflict poped up.
It was saying that item ID "x" is already occupied by railcraft.dust while adding railcraft.dust, this happens regardless of what free ID you set it to.
Which means that Railcraft registers the item twice on the same ID.

Since they are the same it should not break the game in any way.
But it is an error to be noted nevertheless.

Impossible, the dust constructor can only be called once. o.O

Logs.

It only seems to happen with rc dusts installed.
Whole log was to big for pastebin:

2013-11-16 13:41:45 [INFO] [STDOUT] CONFLICT @ 7321 item slot already occupied by mods.railcraft.common.items.ItemDust@154f4b4e while adding mods.railcraft.common.items.ItemDust@8d8f8d0
2013-11-16 13:41:45 [INFO] [fml.ItemTracker] The mod rcdusts is overwriting existing item at 7577 (railcraft.dust from Railcraft) with mods.railcraft.common.items.ItemDust
2013-11-16 13:41:45 [FINE] [fml.ItemTracker] Adding item mods.railcraft.common.items.ItemDust(7577) owned by rcdusts

Because it is replaced by rcdusts immediately it does not matter much
Sorry for blaming you before.

I would like to know what happens there anyway, please explain. :S

That's whahaha's thing right? You'd have to ask him. I have no idea. =P