In C/C++, is there a directive similar to #ifndef for typedefs?

No there is no such facility in C++ at preprocessing stage. At the max can do is

#ifndef thing_type
#define thing_type uint32_t 
#endif

Though this is not a good coding practice and I don't suggest it.

C++ does not provide any mechanism for code to test presence of typedef, the best you can have is something like this:

#ifndef THING_TYPE_DEFINED
#define THING_TYPE_DEFINED
typedef uint32_t thing_type 
#endif

EDIT:
^{As @David, is correct in his comment, this answers the how? part but importantly misses the why? It can be done in the way above, If you want to do it et all, but important it you probably don't need to do it anyways, @David's answer & comment explains the details, and I think that answers the question correctly.}

There is no such thing in the language, nor is it needed. Within a single project you should not have the same typedef alias referring to different types ever, as that is a violation of the ODR, and if you are going to create the same alias for the same type then just do it. The language allows you to perform the same typedef as many times as you wish and will usually catch that particular ODR (within the same translation unit):

typedef int myint;
typedef int myint;       // OK: myint is still an alias to int
//typedef double myint;  // Error: myint already defined as alias to int

If what you are intending to do is implementing a piece of functionality for different types by using a typedef to determine which to use, then you should be looking at templates rather than typedefs.