kdb+ tests to check we're not flipping back and forth - kdb

Question

How do we find out if the sym is flipping back and forth or not?
Once the syms changes to some other sym, it should not go back to previously occurred symbols in history.
Everytime the symbols flips/changes to new symbols, then new symbol should be unique and should not occurred in previous sym.
Things we want here is:
We want to have a Boolean column 1b if the rollover happened
We want to have another Boolean column 1b is rollover happned and the sym has flipped back and forth. (the current sym has occurred in previous or future date syms.)
//To create a sample table
tmp:([]sdate:`date$();sym:`symbol$();name:`symbol$());
{`tmp insert (x;`VXV2;`someName1)} each tdate;
{`tmp insert (x;`VXJ2;`someName2)} each tdate;
{`tmp insert (x;`VXG8;`someName3)} each tdate;
{`tmp insert (x;`VXZ4;`someName4)} each tdate;
//update the dataset to get the scenario
tmp:`sdate`sym xasc tmp;
tmp:update sym:`VXG8 from tmp where sdate=2020.04.11;
tmp:update sym:`VXN6 from tmp where sdate>=2020.04.21;
//the below query is we are interested in to find the rollover and sym flipping back and forth.
select from tmp where name=`someName1
//notes
from 2020-03-30 till 2020-04-10 then sym is VXV2
from 2020-04-11 till 2020-04-11 then sym is VXG8 // the sym was updated to VXG8
from 2020-04-12 till 2020-04-20 then sym is VXV2 // the sym was flipped back to VXV2 from VXG8
from 2020-04-21 till 2021-03-29 then sym is VXN6 // the sym was updated to VXN6

Answer 1

You can use differ on a list to return where it changes. For your second column, you can also use differ and filter out elements that are the first occurrence in the list.
q)list:`A`A`A`A`B`A`A`A`C`C
q)differ list
1000110010b
q)differ[list] and not #[count[list]#0b; list?distinct list; :; 1b]
0000010000b

Answer 2

It depends on exactly how you want the booleans to look - only one true boolean on the flip or all true booleans for every duplicate sym - but this does the former:
update roll:differ sym,dup:differ[sym]&not i=group[sym][;0]sym from select from tmp where name=`someName1
Similar to Cathans answer