You must be playing quick play or QPC as Horizon and Oasis will not show up in the Season 19 Competitive map pool (which is now limited to 12 maps. However I will point out that since the introduction of Role Queue the matchmaker no longer accounts for your recently played maps to find a suitable match for you, as such it is possible to get duplicate maps across multiple matches. The ability to veto maps would overall slow down your ability to find a match and lower the gameplay experience.
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