More than one set of metadata allowed for one local variable (ccpp_prebuild)

[grantfirl]

@mdtoy asked a question related to more than one set of metadata corresponding to one actual variable here: NCAR/ccpp-physics#647

While I think that this particular error is harmless, similar errors might not be. For example, let's say variable X defined in the host has two sets of metadata associated with it with standard names var_X and var_Y. If both var_X and var_Y are referenced from ccpp-physics schemes, I'm guessing that ccpp_prebuild just associates both standard names with one piece of memory so that schemes using either var_X or var_Y can "see" and/or modify it. If someone goes to debug a suite that has physics schemes that use both var_X and var_Y, but they don't know that the variable is referred to be 2 standard names, they may miss the variable being modifying using the standard name that they don't know about. Maybe this problem is far-fetched, but should we add some logic in ccpp_prebuild to disallow more than one set of metadata associated with the same local variable? Or, should this just be a known issue and make sure that capgen does the right thing?