OffsetCurve does not handle repeated points

[dr-jts]

The OffsetCurve class does not handle linework with repeated points correctly. This is due to the raw offset curve generator not handling repeated points correctly. The fix is simply to remove repeated points before generating the raw curve.

Example

Input with repeated point at (9 5):

LINESTRING (4 9, 1 2, 9 5, 9 5, 4 9)

The result for offset distance = 1 is:

if the repeated point is removed, the result is correct:

Originally reported in qgis/QGIS#51750.