Here is the function that perform this task and a results:

from docx import Document


def merge_table_columns_with_equal_text(table, column_index):
    """
    Merge cells in a specific column of a given table only if the value of the previous row cell
    is the same as the value of the current row cell. Only leaves one value in the merged cell.

    :param table: The table object from python-docx
    :param column_index: The index of the column to merge
    """
    num_rows = len(table.rows)
    start_row = None
    start_row_value = None

    for i in range(1, num_rows):
        current_cell = table.cell(i, column_index)
        previous_cell = table.cell(i - 1, column_index)
        current_value = current_cell.text
        previous_value = previous_cell.text
        if previous_value == current_value:
            if start_row is None:
                start_row = i - 1
                start_row_value = table.cell(start_row, column_index).text
            if i + 1 == num_rows:  # if this is the last row
                table.cell(start_row, column_index).merge(table.cell(i, column_index))
                table.cell(start_row, column_index).text = start_row_value
        elif start_row is not None:
            table.cell(start_row, column_index).merge(table.cell(i - 1, column_index))
            table.cell(start_row, column_index).text = start_row_value
            start_row = None


def create_test_document():
    data = [
        ('a', '1', 'x'),
        ('a', '1', 'y'),
        ('a', '1', 'y'),
        ('b', '2', 'z'),
        ('b', '3', 'z'),
        ('c', '3', 'z'),
        ('c', '3', 'z'),
    ]

    document = Document()
    table = document.add_table(rows=len(data), cols=len(data[0]))
    for row_idx, row_data in enumerate(data):
        for col_idx, value in enumerate(row_data):
            table.cell(row_idx, col_idx).text = value

    return document


if __name__ == '__main__':
    doc = create_test_document()
    table = doc.tables[0]
    doc.save('z_test_1_before_merging.docx')
    for column_index in range(0, table._column_count):
        merge_table_columns_with_equal_text(table, column_index)
    doc.save('z_test_2_after_merging.docx')