Is the Indian Constitution plagiarized?
Answer by Dr. Ambedkar himself to the same accusation in the Constituent Assembly Debates.
(https://www.constitutionofindia.net/constitution_assembly_debates/volume/7/1948-11-04)
Dated 04–11–1948
7.48.225 (Volume.Document Number.Paragraph) B.R. Ambedkar It is said that there is nothing new in the Draft Constitution, that about half of it has been copied from the Government of India Act of 1935 and that the rest of it has been borrowed from the Constitutions of other countries. Very little of it can claim originality.
7.48.226 (Volume.Document Number.Paragraph) B.R. Ambedkar One likes to ask whether there can be anything new in a Constitution framed at this hour in the history of the world. More than hundred years have rolled over when the first written Constitution was drafted. It has been followed by many countries reducing their Constitutions to writing. What the scope of a Constitution should be has long been settled. Similarly what are the fundamentals of a Constitution are recognized all over the world. Given these facts, all Constitutions in their main provisions must look similar. The only new things, if there can be any, in a Constitution framed so late in the day are the variations made to remove the faults and to accommodate it to the needs of the country. The charge of producing a blind copy of the Constitutions of other countries is based, I am sure, on an inadequate study of the Constitution. I have shown what is new in the Draft Constitution and I am sure that those who have studied other Constitutions and who are prepared to consider the matter dispassionately will agree that the Drafting Committee in performing its duty has not been guilty of such blind and slavish imitation as it is represented to be.
7.48.227 (Volume.Document Number.Paragraph) B.R. Ambedkar As to the accusation that the Draft Constitution has produced a good part of the provisions of the Government of India Act, 1935, I make no apologies. There is nothing to be ashamed of in borrowing. It involves no plagiarism. Nobody holds any patent rights in the fundamental ideas of a Constitution. What I am sorry about is that the provisions taken from the Government of India Act, 1935, relate mostly to the details of administration. I agree that administrative details should have no place in the Constitution. I wish very much that the Drafting Committee could see its way to avoid their inclusion in the Constitution. But this is to be said on the necessity which justifies their inclusion. Grote. the historian of Greece, has said that: "The diffusion of constitutional morality, not merely among the majority of any community but throughout the whole, is the indispensable condition of a government at once free and peaceable; since even any powerful and obstinate minority may render the working of a free institution impracticable, without being strong enough to conquer ascendency for themselves."
7.48.228 (Volume.Document Number.Paragraph) B.R. Ambedkar By constitutional morality Grote meant "a paramount reverence for the forms of the Constitution, enforcing obedience to authority acting under and within these forms yet combined with the habit of open speech, of action subject only to definite legal control, and unrestrained censure of those very authorities as to all their public acts combined too with a perfect confidence in the bosom of every citizen amidst the bitterness of party contest that the forms of the Constitution will not be less sacred in the eyes of his opponents than in his own."
7.48.229 (Volume.Document Number.Paragraph) B.R. Ambedkar While everybody recognizes the necessity of the diffusion of Constitutional morality for the peaceful working of a democratic Constitution, there are two things interconnected with it which are not, unfortunately, generally recognized. One is that the form of administration has a close connection with the form of the Constitution. The form of the administration must be appropriate to and in the same sense as the form of the Constitution. The other is that it is perfectly possible to pervert the Constitution, without changing its form by merely changing the form of the administration and to make it inconsistent and opposed to the spirit of the Constitution. It follows that it is only where people are saturated with Constitutional morality such as the one described by Grote the historian that one can take the risk of omitting from the Constitution details of administration and leaving it for the Legislature to prescribe them. The question is, can we presume such diffusion of Constitutional morality? Constitutional morality is not a natural sentiment. It has to be cultivated. We must realize that our people have yet to learn it. Democracy in India is only a top-dressing on an Indian soil, which is essentially undemocratic.
7.48.230 (Volume.Document Number.Paragraph) B.R. Ambedkar In these circumstances, it is wiser not to trust the Legislature to prescribe forms of administration. This is the justification for incorporating them in the Constitution.
Analysis
Yes, a large part of the Constitution is a reproduction of the Government of India Act, 1935, multiple features were included in it during the making/copy-pasting of the Constitution.
Features like Parliamentary Government and Bicameralism, Cabinet System, Single Citizenship, Fundamental Rights, Preamble, Fundamental Duties, Union List, Concurrent List, Independent Judiciary and almost all of the rest of the constitution is a copy-paste job from the constitution of multiple foreign countries.
It is not “inspired” in any sense of the word and is a complete and blatant form of plagiarism. It is also the most complicated and the longest constitution ever written, making it extremely complex and leaving ample room for political maneuvering.
The noticeable part here is that the “Draft Constitution” was prepared in a mere 141 days. It was the “deliberations” that took the majority of the time (Read “what to keep and what to delete”). The Indian constitution is not an original piece of work by any stretch of the mind, any person who thinks so is delusional.
I would conclude by saying that it is a copy-paste job without a shred of doubt but it has been made to “appear” novel by the virtue of many-many deliberations that took place before the draft got final approval thereby making it “legal plagiarism”.

“The people made the Constitution, and the people can unmake it. It is the creature of their will, and lives only by their will.” ― John Marshall
Be a proud citizen, of a magnificent Constitution.
Jai Hind!