dup column name crash

run this line of scala:
```
new TableDisplay(Seq(Seq(6,5,8)), Seq("a", "b", "b"), Seq("double", "double", "double"))
```
get this JS crash:
```
Uncaught (in promise) Error: Command 'column_b' already registered.
at CommandRegistry.addCommand (index.js:121)
at IndexMenu.HeaderMenu.addCommand (HeaderMenu.ts:156)
at IndexMenu.HeaderMenu.createItems (HeaderMenu.ts:143)
at IndexMenu.HeaderMenu.createSubmenu (HeaderMenu.ts:201)
at IndexMenu.HeaderMenu.createItems (HeaderMenu.ts:149)
at IndexMenu.buildMenu (IndexMenu.ts:32)
at IndexMenu.HeaderMenu (HeaderMenu.ts:63)
at new IndexMenu (IndexMenu.ts:21)
at DataGridColumn.createMenu (DataGridColumn.ts:112)
at new DataGridColumn (DataGridColumn.ts:89)
```