Learning mexican history is nice to see that during the independence war one of the first things Miguel Hidalgo did was to ban slavery.
But then other mexicans leaders are also mentioned to have abolished it, like José María Morelos, Vicente Guerrero and Guadalupe Victoria.
I even remember reading about mexican presidents abolishing slavery early in the 20th century.
All of this makes me think it that maybe slavery was reinstated?, or were people finding legal loopholes?, or the bans only applied to some places?, or was it simply a move to score political points?
While in paper slavery of indigenous populations was outlawed in colonial Mexico in 1530, under pressure from colonists and the church, who had gathered quite a fortune by "saving" the souls of countless people through -we have a phrase for it in Mexico "voluntariamente a fuerzas"- "voluntary" back-breaking labor in labor cam... err, haciendas and encomiendas the law was... revised in 1534 which allowed specific circumstances under which slave labor was allowed (I should point out that this was opossed by colonial authorities, particularly by people like Vasco de Quiroga), this "special circumstances" were so numerous and the law so lax however that in practice this allowed the institution of slavery to remain as a legality until the Independence of Mexico (and arguably as a reality for a long time after that), and it is important to point out, this (sort of) outlawed slavery of indigenous populations, not of Africans/black people, which continued to be legal until the Independence.
It should be noted as well, that the population of black slaves in Mexico was drastically reduced when the rebellion of Gaspar Yanga showed colonial authorities how dangerous it was to keep a large population of enslaved blacks (so much so, that colonial authorities had to actually recognize the first "free" municipality in the America's history: San Lorenzo de los Negros, latter renamed as: Yanga, in the current day state of Veracruz) in the country (particularly when indigenous labor was so much cheaper), but this didnt eliminate this abominable practice, it just reduced it, in fact here is a 1789 Ruleset for slave trading within the spanish colonies.
By 1810 the practice had admitedly receded in favor of what we would call wage-slaves (people who were paid a misery in exchange for their services), but encomiendas were still capable of forcing groups of people into hard labor, and it wasnt uncommon for the wealthy and powerful to buy black slaves and keep them as what could be called house slaves, often as a way to demostrate wealth, sometimes even bought from British merchants who had outlawed slave traffic in the Empire, but not slavery itself by 1807.
Once Independence kicked of, it took almost 10 years for the insurgents to obtain victory, many of the declarations you are seeing are that, declarations, that were then formalized at other point or other, for example, Hidalgo originally proclaims the abolition of slavery in 1810, but then he has the bad taste of dying and the war goes on for 10 years which is why you find it then officially outlawed in 1824, by the nation of Mexico, not proclaimed by Hidalgo the Spanish rebel, slavery then had to be outlawed repeatedly by the fledging Mexican nation as the practice would rear its hugly head from time to time or would be abolished, but its practice not observed, or had this or that loophole, to my knowledge, baring, its standard re-inclusions (which might be what you are thinking of when you are studying the 20th century) in the 1857 and 1917 Constitutions, the last "formal" abolishment of slavery was in 1837 with a brief period during the French Intervention in which Maximiliano de Habsburgo would reinstate it to gain support from the Confederate States of America... this is all of course ignoring de-facto slavery states like the "Raya" Stores of the Porfirian period, but thats another subject.
As to the multiple abolishments: This has mostly to do with the tumoultus independence wars. The earlier pronouncements by Hidalgo, Morelos and Guerrero all not being carried through since they were killed/died during or shortly after the war. Note that esp. Morelo's pronouncement went much further regarding indigenous and African people's rights than those of most contemporaries. So that Victoria's abolishment was the first official one (as I mention in the post above).
I haven't come across abolishments in the early 20th c. yet, but those would have been quite empty gestures - with at least nominal slavery playing only a very small role even by the early 19th c. In contrast to continuing forms of other labor which came quite close to slavery.