disk table supports duplicate key?

**Describe the feature you'd like**

One big difference between disk table and mem table is that disk table only allows one unique key per index.
For example, if the index is . the rows we inserted are:

|id |col1 | col2|
|-------| ------ | ----- |
1 | k1 | 1
2 | k1 | 1
3 | k2 | 1
4 | k2 | 2

In mem table, it will be:
|id |col1 | col2|
|-------| ------ | ----- |
1 | k1 | 1
2 | k1 | 1
3 | k2 | 1
4 | k2 | 2

However, in disk table, it will be:
|id |col1 | col2|
|-------| ------ | ----- |
1 | k1 | 1
3 | k2 | 1
4 | k2 | 2

because row 1 and row 2 have the same value of .

We should discuss whether we should support the same semantics for both disk table and memory table.

**Additional context**
One possible solution is to add a sequence number to the combined key for disk table.