Please Prepay First

There is a gas station not too far from where I live. On the pump it reads PLEASE PREPAY FIRST. What exactly is going on here? At first I figured that this must be some kind of oversight. With the adaption of "prepay" in most stations, I thought that maybe somebody just changed pay to prepay to follow suit, but forgot to read the finished product. But maybe what is happening is that the first is implying that the first step is to prepay. This seems somewhat plausible, but it then renders the pre- prefix sort of meaningless, since you could just then Please Pay First to convey that meaning. The only thing that I can think of, however likely it may be, is that this is an instance of reduplication of semantic content as an emphatic.

This type of thing can happen with repetition of words Your friend is an ugly ugly man or with syllables as in the Spanish Su amigo es feisisimo (Your friend is very very ugly), or even with the extention of a single phoneme as in Your friend is u-u-ugly, but I can't actually think of an occurence where I have seen this specific type of phenomenon.

The closest that I can produce would be something like a double negative for emphasis, which occurs in some people's speech in English. This would be something like I don't know nobody here, where the nobody is not obligatory, as it is in some dialects. In that case we have two distinct elements, don't and nobody which have the semantic value of negative and one merely adds to the other. The other I came up with was reduplication, of all words, which, according to my etymological dictionary, is composed of a prefix that means "again" and a root that seems to also have semantic value for "again." In other words, why isn't it just called duplication?

In any event, I think it was just a little blunder on somebody's part, but maybe the clerk was just sick and tired of people not paying in advance.