Why are typedef identifiers allowed to be declared multiple times?

Question

From the C99 standard, 6.7(5):

A declaration specifies the interpretation and attributes of a set of identifiers. A definition of an identifier is a d

Answer 1

For the same reason that other declarations are allowed to be declared multiple times. For example:

void foo(int a);

void foo(int a);

int main()
{
    foo(42);
}

void foo(int a)
{
    printf("%d\n", a);
}

This allows more than one header to declare a function or structure, and allow two or more such headers to be included in the same translation unit.

typedefs are similar to prototyping a function or structure -- they declare identifiers that have some compile time meaning, but no runtime meaning. For example, the following wouldn't compile:

int main()
{
    typedef int x;
    typedef int x;
    x = 42;
}

because x does not name a variable; it is only a compile time alias for the name int.