Hey,
By running Build Level I get the mentioned alert in the Topic.
How can I locate my block 0 and 5?

In WED, go to the find object thing in the menu then type in your block number. I don't think that there is such a thing as block 0 though....

From the main menu in WED, go Edit/Find Object and enter the block number.

Ok that one solved by removing one of them. Now I have duplicate 19 and 64.
The 19 is one of the hollow cube blocks around my level and 64 is a small plate somewhere else.

What should I do with these two?

If the hollow cube thing is grouped up and the search points to it, select that group and scope down. Repeat the search. If it's still within a group, keep repeating until you get the individual block you need to "unduplicate".

You have to delete them till there's no other duplicate block.

old_bill

I don't understand the meaning of unduplicate, You mean I should delete it?

What is the point of this warning? It means that you have an object copied twice and they are at the same position?

Yes, it means exactly that.
Maybe you have asigned a key+mouse combination to the copy command,
and when you press this key by accident, it makes an unwanted copy of the block.

old_bill

Usually, when you have duplicate blocks, they are caused from a failed copy operation such as copying something but then having the block positioned in the spot it was copied from as from a rejected action.

Do you know what the prefix "un" means? Duplicate means to make two of something. Un, as in undo, in "unduplicate" means to undo the duplication or simply erase the duplicate.

The point of the warning is simple. Basically, the blocks are in the same space (likely using the same texture) and just wastes memory, disk space, video memory, and causes the game to run slower from having to render the extra block. It's an important warning that you should look into fixing. I've had this once before, but I don't build with blocks anymore.

I was thinking the same as you mentioned but the interesting point is that the block 19 and the block 64 have no common position at all. One of them is as big as my level field and the other one as small as a stair. When you search for those blocks they get red so it shows you have selected them. One of the is small and the other one is way bigger that to be considered as a duplication of the first one.
That I asked what I should do. Is this a bug?