data leakage in `webis/conclugen` dataset
## Describe the bug
Some samples (argument-conclusion pairs) in the *training* split of the `webis/conclugen` dataset are present in both the *validation* and *test* splits, creating data leakage and distorting model results.
Furthermore, all splits contain duplicate samples.

## Steps to reproduce the bug
```python
from datasets import load_dataset

training = load_dataset("webis/conclugen", "base", split="train")
validation = load_dataset("webis/conclugen", "base", split="validation")
testing = load_dataset("webis/conclugen", "base", split="test")

# collect which sample id's are present in the training split
ids_validation = list()
ids_testing = list()

for train_sample in training:
    train_argument = train_sample["argument"]
    train_conclusion = train_sample["conclusion"]
    train_id = train_sample["id"]
    
    # test if current sample is in validation split
    if train_argument in validation["argument"]:
        for validation_sample in validation:
            validation_argument = validation_sample["argument"]
            validation_conclusion = validation_sample["conclusion"]
            validation_id = validation_sample["id"]
            if train_argument == validation_argument and train_conclusion == validation_conclusion:
                ids_validation.append(validation_id)
    
    # test if current sample is in test split
    if train_argument in testing["argument"]:
        for testing_sample in testing:
            testing_argument = testing_sample["argument"]
            testing_conclusion = testing_sample["conclusion"]
            testing_id = testing_sample["id"]
            if train_argument == testing_argument and train_conclusion == testing_conclusion:
                ids_testing.append(testing_id)
```

## Expected results
Length of both lists `ids_validation` and `ids_testing` should be zero.

## Actual results
Length of `ids_validation` = `2556`
Length of `ids_testing` = `287`

Furthermore, there seems to be duplicate samples in (at least) the *training* split, since:
`print(len(set(ids_validation)))` = `950`
`print(len(set(ids_testing)))` = `101`

All in all, around 7% of the samples of each the *validation* and *test* split seems to be present in the *training* split.

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.18.4
- Platform: macOS-12.3.1-arm64-arm-64bit
- Python version: 3.9.10
- PyArrow version: 7.0.0