Someone said that in a YouTube Comments that Devi Durga had problems with a demon that only created clones. I don't know if this is true. What is the story related to it?
Devi Mahatmyam, which is a part of Markandeya Purana doesn't tell that Devi had any problem killing the Ratkabija. You can read the full story in this chapter. Actually the Gods (devas) were greatly afraid seeing the alarming increase in the number (clones) of Raktabija:
व्याप्तमासीत्ततो देवा भयमाजग्मुरुत्तमम्॥८८.५१
Asura’s blood pervaded the whole world; thereat the gods fell into the utmost terror
Seeing this fear of Gods, Sri Chandika just laughs and tells Goddess Kali the way to end that demon:
तान् विषण्णान् सुरान् दृष्ट्वा चण्डिका प्राह सत्वरा । उवाच कालीं चामुण्डे विस्तीर्णं वदनं कुरु॥८८.५२॥ मच्छस्त्रपातसम्भूतान् रक्तबिन्दून् महासुरान् । रक्तबिन्दोः प्रतीच्छ त्वं वक्त्रेणानेन वेगिना॥८८.५३॥ भक्षयन्ती चर रणे तदुत्पन्नान् महासुरान् । एवमेष क्षयं दैत्यः क्षीणरक्तो गमिष्यति । भक्ष्यमाणास्त्वया चोग्रा न चोत्पत्स्यन्ति चापरे॥८८.५४॥
Seeing the gods dejected, Chaṇḍika laughed and said to Kali, “O Chamuṇḍa! stretch out your mouth wide; with this mouth do you quickly take in the great Asuras, which are the drops of blood, that have come into being out of Raktavija at the descent of my weapon on him. Roam about in the battle, devouring the great Asuras who sprang from him; so shall this Daitya with his blood ebbing away meet destruction. These fierce demons are being devoured by thee and at the same time no others will be produced.
Then this is how he met with his end:
इत्युक्त्वा तां ततो देवी शूलेनाभिजघान तम् । मुखेन काली जगृहे रक्तबीजस्य सोणितम्॥८८.५५॥ ततोऽसावाजघानाथ गदया तत्र चण्डिकाम् । न चास्या वेदनां चक्रे गदापातोऽल्पिकामपि॥८८.५६॥ तस्याहतस्य देहात्तु बहु सुस्त्राव शोणितम् । यतस्ततः स्ववक्त्रेण वामुण्डा सम्प्रतीच्छति॥८८.५७॥ मुखे समुद्गता येऽस्या रक्तपातान्महासुराः । तांश्चखादाथ चामुण्डा पपौ तस्य च शोणितम्॥८८.५८॥ देवी शूलेन चक्रेण बाणैरसिभिरृष्टिभिः । जघान रक्तबीजं तं चामुण्डापीतशोणितम्॥८८.५९॥ स पपात महीपृष्ठे शस्त्रसंहतितो हतः । नीरक्तश्च महीपाल रक्तबीजो महासुरः॥८८.६०॥ ततस्ते हर्षमतुलमवापुस्त्रिदशा नृप । तेषां मातृगणो जातो ननर्तासृङ्मदोद्धतः॥८८.६१॥
Having enjoined her thus, the goddess next smote him with her dart. Kali swallowed Raktavjja’s blood with her mouth. Then he struck Chaṇḍika with his club there; and the blow of his club caused her no pain, even the slightest, but from his stricken body blood flowed copiously, and from whatever direction it came, Chamuṇḍā takes it then with her mouth. The great Asuras, who sprang up from the flow of blood in her mouth, Chamunda both devoured them and quaffed his blood. The goddess smote Raktavija with her dart, her thunder-bolt, arrows, swords and spears, when Chamuṇḍā drank up his blood. Stricken with that multitude of weapons, he fell on the earth’s surface, and the great Asura Raktavija became blood-less, O king. Thereat the thirty gods gained joy unparalleled, O king. The band of Mothers which sprang from them broke into a dance, being intoxicated with blood.
Previously here is how he has creating havoc in the Universe:
पलायनपरान् दृष्ट्वा दैत्यान् मातृगणार्दितान् । योद्धुमभ्याययौ क्रुद्धो रक्तबीजो महासुरः॥८८.३९॥ रक्तबिन्दुर्यदा भूमौ पतत्यस्य शरीरतः । समुत्पतति मेदिन्यां तत्प्रमाणो महासुरः॥८८.४०॥ युयुधे स गदापाणिपरिन्द्रशक्त्या महासुरः । ततश्चैन्द्री स्ववज्रेण रक्तबीजमताडयत्॥८८.४१॥ कुलिशेनाहतस्याशु बहु सुस्त्राव शोणितम् । समुत्तस्थुस्ततो योधास्तद्रूपास्तत्पराक्रमाः॥८८.४२॥ यावन्तः पतितास्तस्य शरीराद्रक्तबिन्दवः । तावन्तः पुरुषा जातास्तद्वीर्यबलविक्रमाः॥८८.४३॥ ते चापि युयुधुस्तत्र पुरुषा रक्तसम्भवाः । समं मातृबिरत्युग्रशस्त्रपातातिभीषणम्॥८८.४४॥ पुनश्च वज्रपातेन क्षतमस्य शिरो यदा । ववाह रक्तं पुरुषास्ततो जाताः सहस्रशः॥८८.४५॥ वैष्णवी समरे चैनं चक्रेणाभिजघान ह । गदया दाडयामास ऐन्द्री तमसुरेश्वरम्॥८८.४६॥ वेष्णावीचक्रभिन्नस्य रुधिरस्त्रावसम्भवैः । सहस्रशौ जगद्व्याप्तं तत्प्रमाणैर्महासुरैः॥८८.४७॥ शक्त्या जघान कौमारो वाराही च तथासिना । माहेश्वरी त्रिशूलेन रक्तबीजं महासुरम्॥८८.४८॥ स चापि गदया दैत्यः सर्वा एवाहनत् पृथक् । मातः कोपसमाविष्टो रक्तबीजो महासुरः॥८८.४९॥ तस्याहतस्य बहुधा शक्तिशूलादिभिर्बुवि । पपात यो वै रक्तौघस्तेनासञ्छतशोऽसुराः॥८८.५०॥ तैश्चासुरासृक्सम्भूतैरसुरैः सकलं जगत् ।
Seeing the asuras harassed by the band of Matrs and fleeing, the great asura Raktabija strode forward to fight in wrath. Whenever from his body there fell to the ground a drop of blood, at that moment rose up from the earth asura of his stature. The great asura fought with Indra's shakti with club in his hand; then Aindri also struck Ranktabija with her thunderbolt. Blood flowed quickly and profusely from him who was wounded by the thunderbolt. From the blood rose up (fresh) combatants of his form and valour. As many drops of blood fell from his body, so may persons came into being, with his courage, strength and valour. And those persons also sprung up from his blood fought there with the Matrs in a more dreadful manner hurling the very formidable weapons. And again when his head was wounded by the fall of her thunder-bolt, his blood flowed and there from were born persons in thousands. Vaisnavi struck him with her discus in the battle, Aindri beat that lord of asuras with her club. The world was pervaded by thousands of great asuras who were of his stature and who rose up from the blood that flowed from him when cloven by the discus of Vaisnavi. Kaumari struck the great asura Raktabija with her spear, Varahi with her sword, and Mahesvari with her trident. And Raktabija, that great asura also, filled with wrath, struck everyone of the Matrs severally with his club. From the stream of blood which fell on the earth from him when he received multiple wounds by the spears, darts and other weapons, hundreds of asuras came into being. And those asuras that were born from the blood of Raktabija pervaded the whole world; the devas got intensely alarmed at this.
Well the story you are talking about is the story of a demon named Raktabīja, he came to known by this name after he was blessed with a unique boon by Lord Brahma. As per this boon, if a drop of blood from his body would fall on the ground, clones would be formed out of that drop. The story goes as follows:
Raktabīja was an asura (loosely translated as demon) who fought with Shumbha and Nishumbha against Goddess Parvati and Goddess Kali or Goddess Chamunda. Raktabīja had a boon that whenever a drop of his blood fell on the ground, a duplicate Raktabīja would be born at that spot (rakta = blood, bīja = seed; "he for whom each drop of blood is a seed"). According to some sources, Raktabīja was, in his previous birth, Rambha (asura), king of demons and the father of Mahishasura.
The eighth chapter of the Devi Mahatmya, raktabIja-vadh, focuses on Ambika's battle with Raktabīja as part of her battle against the asuras Shumbha and Nishumbha, who had disenfranchised the gods from heaven. Raktabīja was wounded, but drops of blood falling on the ground created innumerable other Raktabījas, and Ambika and the Matrikas were in difficulty. At this point, the Goddess Kali joined the battle, who collected blood pouring from Raktabīja's body in a bowl while other goddesses wounded him. Kali devoured his duplicates into her gaping mouth. This form who drank the demon's blood is also called Raktheshwari.
Ultimately, Raktabīja was annihilated.