In the Mahabharata Gandhari curses Krishna with the death of all the Yadavas. A rishi cursed Samba to destroy his entire race.
Who is the rishi, why two curses, and whose curse came true?
Section 25 of Stri-Parva gives details of Gandhari's curse.
"Gandhari said, ‘The Pandavas and the Dhartarashtras, O Krishna, have both been burnt. Whilst they were thus being exterminated, O Janardana, why wert thou indifferent to them? Thou wert competent to prevent the slaughter, for thou hast a large number of followers and a vast force. Thou hadst eloquence, and thou hadst the power (for bringing about peace). Since deliberately, O slayer of Madhu, thou wert indifferent to this universal carnage, therefore, O mighty-armed one, thou shouldst reap the fruit of this act.
By the little merit I have acquired through waiting dutifully on my husband, by that merit so difficult to attain, I shall curse thee, O wielder of the discus and the mace! Since thou wert indifferent to the Kurus and the Pandavas whilst they slew each other, therefore, O Govinda, thou shalt be the slayer of thy own kinsmen!
In the thirty-sixth year from this, O slayer of Madhu, thou shalt, after causing the slaughter of thy kinsmen and friends and sons, perish by disgusting means in the wilderness. The ladies of thy race, deprived of sons, kinsmen, and friends, shall weep and cry even as these ladies of the Bharata race!’"
The Section I of the Mausala Parva of Mahabharata gives details of the sages curse.
The rishis that cursed Samba were Vishwamitra Kanva and Narada. Samba and his friends ridiculed the rishis and they cursed the boys in anger.
Vaishampayana continued: "One day, the Vrishni heroes numbering Sarana amongst them, saw Vishvamitra and Kanwa and Narada arrived at Dwaraka.
Afflicted by the rod of chastisement wielded by the deities, those heroes, causing Samva to be disguised like a woman, approached those ascetics and said, ‘This one is the wife of Vabhru of immeasurable energy who is desirous of having a son. Ye Rishis, do you know for certain what this one will bring forth?’
The three rishis, annoyed at the boys trying to decieve them thus curse them as follows:
"Hear now, O king, what those ascetics, attempted to be thus deceived, said:
‘This heir of Vasudeva, by name Samva, will bring forth a fierce iron bolt for the destruction of the Vrishnis and the Andhakas.
Ye wicked and cruel ones, intoxicated with pride, through that iron bolt ye will become the exterminators of your race with the exception of Rama and Janarddana.
The blessed hero armed with the plough will enter the ocean, casting off his body, while a hunter of the name of Jara will pierce the high-souled Krishna while lying on the ground.’
"Endeavoured to be deceived by those wicked ones, those ascetics, with eyes red in wrath, looked at each other and uttered those words.
After this Samba gives birth to an iron pestle. Ugrasena the old king, being terrified of the consequences orders the pestle to be ground into powder and thrown into the sea.
Section III of the same parva details how Yadu race exterminates itself.
Then the Andhakas and the Bhojas, the Saineyas and the Vrishnis, urged by Time, struck one another in that fearful mêlée. Indeed, O king, whoever amongst them took up in wrath a few blades of the Eraka grass, these, in his hands, became soon converted into a thunderbolt, O puissant one. Every blade of grass there was seen to be converted into a terrible iron bolt. All this, know, O king, was due to the curse denounced by Brahmanas.
The same Section describes the fulfillment of Gandhari's curse, that Krishna shall slay his own children.
The delighter of the Yadus, beholding his own son, and the son of Sini too, slain, took up, in wrath, a handful of the Eraka grass that grew there. That handful of grass became a terrible bolt of iron endued with the energy of the thunderbolt. With it Krishna slew all those that came before him.
Section IV details how Krishna and Balarama gave up their incarnations.
"Proceeding then to the forest, Keshava beheld Rama sitting in a solitary spot thereof.
He also saw that Rama had set himself to Yoga and that from out his mouth was issuing a mighty snake. The colour of that snake was white.
Leaving the human body (in which he had dwelt so long), that high-souled naga of a 1,000 heads and having a form as large as that of a mountain, endued besides with red eyes, proceeded along that way which led to the ocean.
Here before leaving his incarnation Krishna even thinks about Gandhari's curse. He meets his end in the forest just as Gandhari had cursed "you will perish in the wilderness". We can also see that he departs in the same way as the sages cursed.
"After his brother had thus departed from the (human) world, Vasudeva of celestial vision, who was fully acquainted with the end of all things, wandered for some time in that lonely forest thoughtfully. Endued with great energy he then sat down on the bare earth. He had thought before this of everything that had been fore-shadowed by the words uttered by Gandhari in former days. He also recollected the words that Durvasas had spoken at the time his body was smeared by that Rishi with the remnant of the Payasa he had eaten (while a guest at Krishna’s house). The high-souled one, thinking of the destruction of the Vrishnis and the Andhakas, as also of the previous slaughter of the Kurus, concluded that the hour (for his own departure from the world) had come.
"A fierce hunter of the name of Jara then came there, desirous of deer. The hunter, mistaking Keshava, who was stretched on the earth in high Yoga, for a deer, pierced him at the heel with a shaft and quickly came to that spot for capturing his prey.
Now as to why two curses it does not say but I'm giving my opinion based answer.
After Gandhari curses Krishna this is what he says in reply.
Vaishampayana continued, "Hearing these words, the high-souled Vasudeva, addressing the venerable Gandhari, said unto her these words, with a faint smile, ‘There is none in the world, save myself, that is capable of exterminating the Vrishnis. I know this well. I am endeavouring to bring it about. In uttering this curse, O thou of excellent vows, thou hast aided me in the accomplishment of that task. The Vrishnis are incapable of being slain by others, be they human beings or gods or Danavas. The Yadavas, therefore shall fall by one another’s hand.’
He says that he is 'endeavouring' or trying to bring about the destruction of his own race. And he also says that Gandhari has 'aided' or helped him in the accomplishment of that task.
Now in Srimad Bhagvatam Canto 11 Chapter 1 there is this verse:
Lord Krsna thought, “No outside force could ever bring about the defeat of this family, the Yadu dynasty, whose members have always been fully surrendered to Me and are unrestricted in their opulence. But if I inspire a quarrel within the dynasty, that quarrel will act just like a fire created from the friction of bamboo in a grove, and then I shall achieve My real purpose and return to My eternal abode.”
My dear King Pariksit, when the supreme almighty Lord, whose desire always comes to pass, had thus made up His mind, He withdrew His own family on the pretext of a curse spoken by an assembly of brahmanas.
Gandhari's curse and the words spoken by her gave him an idea and that was to eliminate his race through a curse that creates a fight amongst his family. Gandhari also says that he he shall cause slaughter of his family which means both in the real sense where Krishna physically kills his children and in the causative sense as well where Krishna ends his family through the sages' curses.
As Narayana says in Shanti Parva here:
I am, O Narada, the Doer, I am Cause, and I am Effect. I am the sum-total of all living creatures.