The reason there were two in the same year is because the provisional government kept Russia in World War I. Opposition to the war was very strong in Russia, and was what drove Nicholas to abdicate. By staying in the war, the provisional government insured that it would never gain widespread support, and gave the Bolsheviks the opportunity to undermine it. As far as why most people think there was only one, people tend to focus on the end result, and the end result of the events of 1917 in Russia was that the Bolsheviks were in control.
The first was the overthrow of the Monarchy in 1917, also known as the February Revolution. This deposed Tsar Nicholas II, but nobody really figured out who should run the country. So, there was a period of dual power, in which the in which the Provisional Government held state power and the national network of Soviets, led by socialists, had the allegiance of the lower-classes and the political left.
The second revolution, known as the October Revolution was when the Bolshevik party, led by Vladimir Lenin, and the workers' Soviets, overthrew the Provisional Government and brought about a massive change in the social structure of Russia, as well as paving the way for the USSR. There were tons of infamous riots and such that happened in Moscow and St. Petersburg, but there was also a huge movement in cities throughout the country, among national minorities throughout the empire, and in the rural areas, where peasants revolted, then seized and redistributed lan
Reasons why 1905 failed was because of the famous "repression and concessions" of Nicholas the II, he conceded to ending redemption payments for the peasants (thus appeasing them), he allowed the Duma to be created and raised wages of workers and soldiers. As you can see he has satisfied major aspects of Russian society (apart from the nobility and church who were declining anyways). In 1917 these powerful groups of people were no longer under the thumb of Nicholas II.
From a socialist theory of History we must firstly look at the economy, inflation was at a disastrous peak which led to food shortages which naturally would have infuriated workers in the cities (as this was where the food crisis was worse arguably). One could even make a comparison to the rise of Hitler who also played on the economic crisis of Germany and hyperinflation. Moreover a long term cause was Russia's total lack of industrialisation, which not only led to economic problems of not being able to the most modern weapons but also meant workers had more labour intensive work and more dangerous in some cases.
Secondly, the provisional government had failed to end the war, which infuriated the people who not only had to deal with losses in their family but also and most