This practice may have been a result of expressing English documents with a mixture of Anglo-Saxon and French, or Latin terms.
When early writers weren't sure if both designations had the same meaning, or that others might not have a clear understanding of the French or Latin, they apparently included terms from both the Anglo-Saxon and the "foreign"; words side by side, just to be sure others understood what was meant. This is according to David Crystal in The Cambridge Encyclopedia of Language.
Pleonasms are the opposites (antonyms) of oxymora. A pleonasm consists of two concepts (usually two words) that are redundant. What does "redundant" mean? Well, how about "more than enough; overabundant; excessive; and superfluous"?
Still having a problem understanding what pleonasm means? Some pleonastic expressions are also known as tautologies. Tautology means, "needless repetition of an idea in a different word, phrase, or sentence; redundancy; pleonasm". All right, what about pleonasm? Well, it means, "the use of more words than are necessary for the expression of an idea; redundancy".
So it is that we go around in circles: pleonasm means tautology, which means redundancy, which means pleonasm, which means tautology, ad infinitum.