Sumation type arugments on interfaces cause compilation issues

[hunterwerlla]

Using Dukat 0.5.7 and Kotlin 1.4.0-rc, generating externals for "@types/vscode": implementation(npm("@types/vscode", "1.42.0", generateExternals = true))

The vscode code:

export interface QuickPickOptions {
...
onDidSelectItem?(item: QuickPickItem | string): any;
...
}

generates:

    val onDidSelectItem: ((item: QuickPickItem) -> Any)?
        get() = definedExternally
    val onDidSelectItem: ((item: String) -> Any)?
        get() = definedExternally

which causes a Conflicting declarations error.