From first glance, it seems as if the LBC of 1689 and the Savoy Declaration are plagiarized copied of the WCF of 1647.
Answer:
From the introduction to the London Baptist Confession:
“Courteous Reader: It is now many years since divers of us (with other sober Christians then living, and walking in the way of the Lord, that we profess) did conceive ourselves to be under a necessity of publishing a Confession, of our Faith, for the information and satisfaction of those that did not thoroughly understand what our principles were, or had entertained prejudices against our profession, by reason of the strange representation of them by some men of note who had taken very wrong measures, and accordingly led others into misapprehension of us and them. And this was first put forth about the year 1643, in the name of seven congregations then gathered in London; since which time divers impressions thereof have been dispersed abroad, and our end proposed in good measure answered, inasmuch as many (and some of those men eminent both for piety and learning) were thereby satisfied that we were no way guilty of those heterodoxies and fundamental errors which had too frequently been charged upon us without ground or occasion given on our part.”
And forasmuch as that Confession is not now commonly to be had, and also that many others have since embraced the same truth which is owned therein, it was judged necessary by us to join together in giving a testimony to the world of our firm adhering to those wholesome principles by the publication of this which is now in your hand. And forasmuch as our method and manner of expressing our sentiments in this doth vary from the former (although the substance of this matter is the same), we shall freely impart to you the reason and occasion thereof. One thing that greatly prevailed with us to undertake this work was (not only to give a full account of ourselves to those Christians that differ from us about the subject of baptism, but also) the profit that might from thence arise unto those that have any account of our labors in their instruction and establishment in the great truths of the Gospel, in the clear understanding and steady belief of which our comfortable walking with God, and fruitfulness before him in all our ways, is most nearly concerned; and therefore we did conclude it necessary to express ourselves the more fully and distinctly; and also to fix on such a method as might be most comprehensive of those things we designed to explain our sense and belief of; and finding no defect in this regard in that fixed on by the Assembly, and, after them by those of the congregational way, we did readily conclude it best to retain the same order in our present Confession; and also when we observed that those last mentioned did in their Confessions (for reasons which seemed of weight both to themselves and others) choose not only to express their mind in words concurrent with the former in sense concerning all those articles wherein they were agreed, but also for the most part without any variation of the terms, we did in like manner conclude it best to follow their example in making use of the very same words with them both in these articles (which are very many) wherein our faith and doctrine are the same with theirs; and this we did the more abundantly to manifest our consent with both in all the fundamental articles of the Christian religion, as also with many others whose orthodox Confessions have been published to the world on the behalf of the Protestant in diverse nations and cities. And also to convince all that we have no itch to clog religion with new words, but do readily acquiesce in that form of sound words which hath been, in consent with the Holy Scriptures, used by others before us; hereby declaring, before God, angels, and men, our hearty agreement with them in that wholesome Protestant doctrine which, with so clear evidence of Scriptures, they have asserted. Some things, indeed, are in some places added, some terms omitted, and some few changed; but these alterations are of that nature as that we need not doubt any charge or suspicion of unsoundness in the faith from any of our brethren upon the account of them.
In those things wherein we differ from others we have expressed ourselves with all candor and plainness, that none might entertain jealousy of aught secretly lodged in our breasts that we would not the world should be acquainted with; yet we hope we have also observed those rules of modesty and humility as will render our freedom in this respect inoffensive, even to those whose sentiments are different from ours.
Tabular comparisons of the WCF vs LBC: http://www.proginosko.com/docs/wcf_lbcf.html