Is there a way to check if a label is already defined in LaTeX? -

I answered the question after the reply of David Hanak (thanks btw!). He helped with syntax, but it seems that I have not been used to start with the right function.

Actually, I want the compiler to ignore several definitions of a certain label and just give it the first use. In order to do this, I thought that I just want to do something like this:

 \ makeatletter \ newcommand {\ mylabel} [1] {\ @ifundefined {# 1} {\ label {# 1 } {X}} \ makeatother 

Although this does not work, because the first option is always chosen (it does not matter if the label is defined or not). I think \ @ifundefined (and suggested \ ifundefined) works only for the command and not for the label, but I do not know much about latex. Any help with this would be great! Thanks!

Update very later: I considered the response to David Hanak as the right answer to my question, but this is not a complete solution, though it really helps me, the problem is, I think, But I am not an expert, although Dawood's code also checks to see whether a label is defined or not, it only works when the label was last defined (ie. In). If two \ mylabels are defined in the same run with the same name, the second will still be defined. Besides, even if you manage to work around this, it will use the first label for latex Which you have defined chronologically, and not necessarily the first in the text. Anyway, there is my quick and dirty solution below. It uses the fact that the counter appears to be defined right away.

 \ newcommand {\ mylabel} [1] {% \ @ifundefined {c @ # 1} {% \ newcounter {# 1}% \ setcounter {# 1} {0}%} {}% \ Ifthenelse {\ value {# 1}> 0} {} {%} Label {# 1}% \ addtocounter {# 1} {1}%}%}% 

I'm not sure that the counter It is necessary to start with 0, because it looks like a potential default, but I do not know that this is the case, so I'm just safe. Also, it uses the 'ifthen' package, which I'm not sure is necessary.

@ is a special character in latex syntax right to make your declaration For, you must add two more lines:

  \ makeatletter \ newcommand {\ mylabel} [1] {\ @ifundefined {# 1} {\ label {# 1}} {X}} \ Makeatother  

replaces the first line @ in a normal letter, the last line reverses its effect.

Update: You can also look at "plain".

Update 2

OK, I researched the answer to the real problem is that the label Defining does not make a macro of that name; This creates an "R @" for it, then try the following:

  \ makeatletter \ newcommand {\ mylabel} [1] {\ @ifundefined {r @ # 1} {\ label For more technical explanation making {# 1}} {X}}, see Line 3863 of Latex.Ltx in your latex distribution (where it is  \ def \ newlabel {\ @ newl @ bel r} >).