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Suppose we have the following list

data = {{0,1},{-1,2},{5,-2},{4,4},{3,0},{-1,1},{6,0.5},{7,2},{8,-5},{9,2}}

How can we remove all elements of this list which have the same x value? For example, in the above example, the entries {-1,2} and {-1,1} should be removed, since in both of them the x value is the same (-1).

I tried DeleteDuplicates[data] but it does not work.

Many thanks in advance and Merry Christmas to all!

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Select[Counts[data[[All, 1]]]@#[[1]] == 1 &] @ data

Join @@ Select[Length@# == 1 &] @ GatherBy[data, First]

Values @ GroupBy[data, First, If[Length @ # > 1, Nothing, #[[1]]] &]

data[[Flatten @ Select[Length @ # == 1 &] @ 
   GatherBy[Range @ Length @ data, data[[#, 1]] &]]]

FixedPoint[SequenceReplace[#, b : {{a_, _}, ___, {a_, _}} :> 
    Sequence @@ DeleteCases[b, {a, _}]] &, data]

all give

 {{0, 1}, {5, -2}, {4, 4}, {3, 0}, {6, 0.5`}, {7, 2}, {8, -5}, {9, 2}}
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Tally[data, First@#1 == First@#2 &] // Cases[{x_, 1} :> x]

gives

{{0, 1}, {5, -2}, {4, 4}, {3, 0}, {6, 0.5}, {7, 2}, {8, -5}, {9, 2}}
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data[[Flatten[Select[Part[#,2]&/@Normal[PositionIndex[First/@data]],Length[#]>1&]]]]={"delThis"};
data=DeleteCases[data,{"delThis"}]

{{0, 1}, {5, -2}, {4, 4}, {3, 0}, {6, 0.5}, {7, 2}, {8, -5}, {9, 2}}
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