Where are you getting that from? And how much does it really affect anything?
The amount of data necessary to define an object in one inventory slot is not all that big. You've got two bytes to define the item number (since there are over 255 different item types) and one byte for the amount (since it's (artificially) limited to 64) for a total of three bytes, not counting how the game keeps track of inventory slots. So that's three extra bytes per item slot. That's so insignificant so as to be totally ignorable.
As for map size, each block is one block type and each chunk will be exactly the same size regardless of what normal blocks are in it. You could have a chunk with nothing but air blocks and a chunk with nothing but bedrock all the way up to the skytop and both chunks will be exactly the same datasize because they are using the same data block#@position# data. Only special blocks with extra variables assigned to them will be the only blocks that have a larger datasize, such as door blocks, sign blocks, torch blocks, and the like. So the only way duplicating would effect a larger map size would but if you made a huge amount of switches or something and placed them every damned place.
All of this is, of course, assuming that the way data is handled works as it seems to me.
The amount of players that are currently in the server faaaaar outstrips any increased memory usage that duplicating would effect.