According to Wikipedia, Lord Rama and Sita have twins named Lava (Luv) and Kusha (Kush). From childhood I have also learnt that Lord Rama had twins. While reading this question, I came across this comment that mentions that Kusha was made by Valmiki as a clone to Lava and that Lord Rama has only one son Lava. Can anyone clarify which of these is true?
Of course he is. The Ramayana says Sita gave birth to two sons, not that Vamiki cloned one from the other:
yāmeva rātriṃ śatrughna parṇaśālāṃ samāviśat
tāmeva rātriṃ sītāpi prasūtā dārakadvayam [VRM - 7.66.1]
On the same night in which Satrughna took shelter in the hermitage, Sita gave birth to two sons.
That same chapter then goes on to describe how the two sons get the name of Lava and Kusha. After the birth of the sons, the disciples there informed Valmiki of this and told to protect the new born children from ghosts and demons.
So Valmiki took a grass (Kusha), cut it off (lava) into two parts and sanctified it with mantras. He instructed the women there to rub the upper part to the elder boy and the lower part to the younger boy. That is how they came to be known as Lava and Kusha. [7.66.4-7]
So some probably take that ritual that Valmiki did to save the new born twins from the effect of ghosts, demons etc. as a cloning process, but it is not. As evident from the scripture, Sita gave birth to two sons, there were no clones made by Valmiki.
Yes, Lava and Kusha are definitely both sons of Rama and Sita. The birth of Lava and Kusha in the ashram of the sage Valmiki is described in this excerpt from the Uttara Kanda of the Ramayana:
On the same night Satrughna housed himself in a thatch-cottage Sita gave birth to twin sons. In the midnight the Muni boys came to Valmiki and communicated unto him is auspicious intelligence, saying "O illustrious Sir, the loved spouse of Rama hath given birth to two sons ; do thou so protect them that evil spirits might not injure them." Hearing aring those words the highly effulgent Valmiki went there and attained to great delight on beholding those two highly effulgent sons, resembling the newly risen sun and the celestials.
Thereupon he made arrangements against the oppression of ghosts and goblins. Thereupon taking Kusa (the upper part of the grass) and Lava (the lower part) the great ascetic Valmiki made those arrangements. And giving Kusa sanctified by Mantras unto the hands of elderly women he said, "do ye rub the person of the elder boy with these" ; and giving them Lava he said "do ye rub the person of the younger brother with this. And according to this I shall name the first Son Kusa and the second Lava ; and by those names they shall be celebrated on earth."
And just so that there's no doubt that Rama is the father, here is what Valmiki tells Rama about the paternity of Lava and Kusha, in another excerpt from the Uttara Kanda of the Ramayana:
I tell you the truth, O Rama, that these irrepressible twin brothers are your sons, O descendant of Raghu. I am the tenth son of Pracheta, so, far from speaking untruth it does not even spring up in my mind. I therefore know the truth that these twins are thy sons.
In Ramayana it's this way that Kusha was born:
Once Sita goes to fetch water from a lake carrying infant Lava in her arms. Sage Valmiki then comes to the ashram from outside and asks where Sita is. The ashram people say she has gone to fetch water with other ladies and he does not see the child in the cradle of Lava. He thinks demons should have taken the baby so he immediately gets some dry grass (Dharba or Kusha in Sanskrit) and clones a baby by the time Sita comes to the ashram, this is how Kusha is born and how he gets the name Kusha.