legal doublet
What is now thought of as the English legal system was Norman in origin and introduced with the Conquest. Naturally, the Norman French of the time was the language of the courts to begin with (law courts, royal courts and manorial courts too), albeit with considerable Latin influence.
Inevitably, English crept in increasingly with each passing generation, and at the same time French declined as a vernacular language in England, even among the nobility. Under Edward III, who was expanding English holdings in France, the English language gained a significant victory in 1362 with the passage of the Statute of Pleading by the English Parliament, which, noting that French was not generally understood among the populace, provided that:
All Pleas which shall be pleaded in [any] Courts whatsoever, before
any of his Justices whatsoever, or in his other Places, or before any
of His other Ministers whatsoever, or in the Courts and Places of any
other Lords whatsoever within the Realm, shall be pleaded, shewed,
defended, answered, debated, and judged in the English language, and
that they be entered and inrolled in Latin.
The Statute of Pleading was by no means a complete win for the English language. French retained its prestige and survived for centuries to come in schools and universities, and, importantly for us, in the Inns of Court (where it held on longest, becoming increasingly ossified and divorced from spoken French and natural language before finally dying, a solely written bastardised shell of its former self). The Statute of Pleading also enshrined Latin as the language of record in the English courts.
So, though English had been given rightful prominence in court proceedings, the multilingual legal environment put in place by the Norman Conquest still stubbornly persisted. One of the enduring legacies of this are the repetitive set phrases called legal doublets.
You have more or less described their origin and pattern you describe when you say they are:
...filler because they are literally just repetition of the same
idea...
and
...such phrases came about when the French and English started living
side-by-side and they would repeat the same thing twice, once in
English and once in French...
Wikipedia gives the following definition for legal doublets:
A legal doublet is a standardized phrase used frequently in English
legal language consisting of two or more words that are near synonyms.
The origin of the doubling — and sometimes even tripling — often lies in
the transition from use of one language for legal purposes to use of
another for the same purposes, as from a Germanic ([Anglo-]Saxon or
Old English) term to a Romance (Latin or Law French) term or, within
the Romance subfamily, from a Latin term to a Law French term. To
ensure understanding, words of Germanic origin were often paired with
words having equivalent or near-equivalent meanings in Latin
(reflecting the interactions between Germanic and Roman law following
the decline of the Roman Empire) or, later, Law French (reflecting the
influence of the Norman Conquest), and words of Latin origin were
often paired with their Law French cognates or outright descendants.
Such phrases can often be pleonasms and Siamese twins.
(Emphasis added.)
Pleonasm refers to the deliberate emphatic tautology (like "blackest night"), and Siamese twin to the phrases' set nature and fixed order (like "knife and fork").
Examples of legal doublets include: over and above, terms and conditions, and goods and chattels. There is a marked tendency towards alliteration, seen in phrases such as aid and abet and have and hold. (The Wikipedia article linked to previously has a list.)
Legal triplets also exist, such as give, devise and bequeath, or sign, seal and deliver.
These phrases (often repetitive and redundant-seeming as they are) continue to influence the voice and idiom of Legal English to this day, wherever the common law has gone. The term full faith and credit from Article IV of the US Constitution, which dates from 1787, and the relatively modern phrase clear and present danger, originating in a 1919 US Supreme Court decision rather than emanating from the King's Bench and the darker recesses of legal history, still both obviously owe something to legal doublets in their form of expression.
Also, the common law being what it is, precedence gives weight to traditional forms, especially in areas such as land law, where established formulae of words have specifically prescribed meanings creating particular forms of interest in real property.
Traditional phrasings persist for reasons of certainty and prudence in other areas too. To this day, wills are populated with these phrases, such as give, devise and bequeath, even when they are not strictly necessary, as in probate matters the common law courts strive to give effect to the wishes of the testator. (Even last will and testament itself is a legal doublet, pairing a Germanic word with a Romance one, although it is of a slightly different origin: originally a will was for real property, and a testament for goods and chattels.)
Note that while legal doublets do seem to be at least one instance of the phenomenon you are describing in your question, the phrase cannot, of course, be used to describe anything other than legal language and there might be a better, more general term.
Having said that, legal doublets are strange beasts indeed, and such structures might not be able survive, never mind thrive, without the peculiar circumstances of the Norman Conquest to incubate them and the artificial and linguistically conservative court environment to coddle them along the way. So it is perhaps only in Legal English that the phrases you describe in your question exist.
Note that the doublet in legal doublet in no way has the same sense as the normal linguistic/etymological sense of doublet, which does not imply a set phrase or even a collocation.
