Doctrine and Covenants 35:17-21
17 And I have sent forth the fulness of my gospel by the hand of my servant Joseph; and in weakness have I blessed him;
18 And I have given unto him the keys of the mystery of those things which have been sealed, even things which were from the foundation of the world, and the things which shall come from this time until the time of my coming, if he abide in me, and if not, another will I plant in his stead.
19 Wherefore, watch over him that his faith fail not, and it shall be given by the Comforter, the Holy Ghost, that knoweth all things.
20 And a commandment I give unto thee—that thou shalt write for him; and the scriptures shall be given, even as they are in mine own bosom, to the salvation of mine own elect;
21 For they will hear my voice, and shall see me, and shall not be asleep, and shall abide the day of my coming; for they shall be purified, even as I am pure.
more:
D&C 42
56 Thou shalt ask, and my scriptures shall be given as I have appointed, and they shall be preserved in safety;
57 And it is expedient that thou shouldst hold thy peace concerning them, and not teach them until ye have received them in full.
58 And I give unto you a commandment that then ye shall teach them unto all men; for they shall be taught unto ball nations, kindreds, tongues and people.
59 Thou shalt take the things which thou hast received, which have been given unto thee in my scriptures for a law, to be my law to govern my church;
60 And he that doeth according to these things shall be saved, and he that doeth them not shall be damned if he so continue.
61 If thou shalt ask, thou shalt receive revelation upon revelation, knowledge upon knowledge, that thou mayest know the mysteries and peaceable things—that which bringeth joy, that which bringeth life eternal.
D&C 82: 4
4 Ye call upon my name for revelations, and I give them unto you; and inasmuch as ye keep not my sayings, which I give unto you, ye become transgressors; and justice and judgment are the penalty which is affixed unto my law.
Since all the revelations of Jesus were written from the foundation of the earth and revealed at times to various prophets any charge of plagiarism is patently absurd. That they are the same from one revelation to another proves a common provenance thus making the revelation more likely to be of God.
I suppose a Bible quote is required.
Rom. 16: 25
25 Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began,
So Paul's CHARITY revelation was kept secret since the world began and just happened to be revealed to Alma before Paul.