Fix duplicated words and a misspelling in messages and docstrings (#14287)

Remove accidental repeated words in eight "must have have batch size"
error messages raised by the modular pipelines, and in two docstrings
("Dimension to to encode", "image batch to be be inpainted"). Also fix
"arbitary" to "arbitrary" in the LTX2 pipeline docstring.

Text only, no behavior change.