Concatenate 2 text measures without duplicates

One approach would be to use "|" instead of ", " as a separator, and then work with the PATHITEM() function in DAX.

Another approach would be to convert your text string into a table variable.

By the way, your table 1 is not required, and neither is your TAGS2 measure. Unless your sample data is not representative?

CALCULATED COLUMN TAGS2:

TAGS2 = SUBSTITUTE('Table 2'[Tags Table 2],", ","|")

Measures:

TAGS3 = CONCATENATEX(values('Table 3'[Tags Table 3]),'Table 3'[Tags Table 3],",")

Measure = 
var t3=SUMMARIZE(ADDCOLUMNS(values('Table 3'[Tags Table 3]),"TAG",'Table 3'[Tags Table 3]),[TAG])
var u=UNION(t3,row("TAG",PATHITEM(SELECTEDVALUE('Table 2'[TAGS2]),1)),row("TAG",PATHITEM(SELECTEDVALUE('Table 2'[TAGS2]),2)))
return CONCATENATEX(DISTINCT(FILTER(u,[TAG]<>BLANK())),[TAG],",")

Note: this is just for illustration - you need to union the other path rows too if there are more than two.

Result: 

Note the data quality issues around SILK and the empty string for XXXX4 (I replaced that with null in the data load).